Fixup android local resolvers.

Previously the android local SDK resolvers used <ibiblio/> with local
file:// roots.  This led to issues resolving any <dependency/> with a
custom artifact file:// url itself.

Simplify ResolveJarsTestMixin which no longer needs a custom
ivysettings.xml as a result of this fix.

Testing Done:
This ran green locally (I have an android setup):
```
$ ./pants clean-all test \
  tests/python/pants_test/android/:: \
  tests/python/pants_test/backend/project_info/::
```

CI went green here:
  https://travis-ci.org/pantsbuild/pants/builds/83968079

Bugs closed: 2324

Reviewed at https://rbcommons.com/s/twitter/r/2934/

build-support/ivy/ivysettings.xml

@@ -5,19 +5,33 @@ Licensed under the Apache License, Version 2.0 (see LICENSE).
 -->
 
 <ivysettings>
-
   <settings defaultResolver="pants-chain-repos"/>
 
-  <properties environment="env" />
   <property name="m2.repo.relpath" value="[organisation]/[module]/[revision]"/>
   <property name="m2.repo.pom" value="${m2.repo.relpath}/[module]-[revision].pom"/>
   <property name="m2.repo.artifact"
             value="${m2.repo.relpath}/[artifact](-[classifier])-[revision].[ext]"/>
   <property name="m2.repo.dir" value="${user.home}/.m2/repository" override="false"/>
 
-  <property name="env.ANDROID_HOME" value="/please-export-your-ANDROID_HOME" override="false"/>
-  <property name="android.repo.dir" value="${env.ANDROID_HOME}/extras/android/m2repository"/>
-  <property name="google.repo.dir" value="${env.ANDROID_HOME}/extras/google/m2repository"/>
+  <!-- This dance to set ANDROID_HOME seems more complex than it need be, but an absolute path is
+       needed whether or not the ANDROID_HOME env var is set and just using the following does
+       not work to default the value when unset in the environment:
+
+       <properties environment="env"/>
+       <property name="env.ANDROID_HOME" value="/please-export-your-ANDROID_HOME" override="false"/>
+
+       -->
+  <properties environment="env"/>
+  <property name="ANDROID_HOME"
+            value="/please-export-your-ANDROID_HOME"
+            unlessset="env.ANDROID_HOME"/>
+  <property name="ANDROID_HOME"
+            value="${env.ANDROID_HOME}"
+            ifset="env.ANDROID_HOME"/>
+
+  <property name="android.repo.dir" value="${ANDROID_HOME}/extras/android/m2repository"/>
+  <property name="google.repo.dir" value="${ANDROID_HOME}/extras/google/m2repository"/>
+
   <resolvers>
 
     <chain name="pants-chain-repos" returnFirst="true">
@@ -27,21 +41,28 @@ Licensed under the Apache License, Version 2.0 (see LICENSE).
            We setup the maven central resolver to require successful pom downloads here. -->
       <ibiblio name="maven-central" m2compatible="true" descriptor="required"/>
 
+      <!-- NB: Although url resolvers (like <ibiblio/>) can be pointed at local file:// system
+           URLs, this leads to issues resolving any <dependency/> with a custom artifact that
+           defines a file:// url itself.  We stick to <filesystem/> resolvers here for the
+           two android SDK resolvers as a result. -->
+
       <!-- Android SDK Support Library, found in reference to the ANDROID_HOME variable. -->
-      <ibiblio name="local.android"
-               m2compatible="true"
-               root="file://${android.repo.dir}/"/>
+      <filesystem name="local.android" m2compatible="true" local="true">
+        <ivy pattern="${android.repo.dir}/${m2.repo.pom}"/>
+        <artifact pattern="${android.repo.dir}/${m2.repo.artifact}"/>
+      </filesystem>
+
       <!-- Google Libraries for the Android SDK, found in reference to ANDROID_HOME. -->
-      <ibiblio name="local.google"
-               m2compatible="true"
-               root="file://${google.repo.dir}/"/>
+      <filesystem name="local.google" m2compatible="true" local="true">
+        <ivy pattern="${google.repo.dir}/${m2.repo.pom}"/>
+        <artifact pattern="${google.repo.dir}/${m2.repo.artifact}"/>
+      </filesystem>
 
       <!-- The mvn standard local filesystem repo/cache -->
       <filesystem name="local.m2" m2compatible="true" local="true" checkmodified="true">
         <ivy pattern="${m2.repo.dir}/${m2.repo.pom}"/>
         <artifact pattern="${m2.repo.dir}/${m2.repo.artifact}"/>
       </filesystem>
-
     </chain>
   </resolvers>
 </ivysettings>

tests/python/pants_test/backend/project_info/tasks/resolve_jars_test_mixin.py

@@ -30,21 +30,6 @@ def _test_jar_lib_with_url(self, load_all):
       with temporary_dir(root_dir=self.workdir_root()) as source_dir:
         with temporary_dir(root_dir=self.workdir_root()) as dist_dir:
           os.makedirs(os.path.join(source_dir, 'src'))
-          ivy_settings = os.path.join(source_dir, 'build-support', 'ivy', 'ivysettings.xml')
-          os.makedirs(os.path.dirname(ivy_settings))
-          with open(ivy_settings, 'w+') as f:
-            # NB(gmalmquist): It is necessary to use ivysettings different from the default to allow
-            # resolution of jars from the local filesystem. (This took quite some time to determine,
-            # because the ivy cache made it seem like resolves were working when they shouldn't,
-            # hence the step to delete the cached artifact generated by this test a couple dozen
-            # lines down).
-            f.write(dedent('''
-              <?xml version="1.0"?>
-              <ivysettings>
-                <settings defaultResolver="default"/>
-                <include url="${ivy.default.settings.dir}/ivysettings.xml"/>
-              </ivysettings>
-            ''').strip())
           with open(os.path.join(source_dir, 'BUILD'), 'w+') as f:
             f.write(dedent('''
               source_root('src')
@@ -100,10 +85,7 @@ def _test_jar_lib_with_url(self, load_all):
           targets = ['{0}:{1}'.format(os.path.join(source_dir, 'src'), name) for name in spec_names]
 
           with temporary_dir() as ivy_temp_dir:
-            extra_args = [
-              '--ivy-ivy-settings={}'.format(ivy_settings),
-              '--ivy-cache-dir={}'.format(ivy_temp_dir),
-            ]
+            extra_args = ['--ivy-cache-dir={}'.format(ivy_temp_dir)]
             self.evaluate_subtask(targets, workdir, load_all, extra_args=extra_args,
                                   expected_jars=['org.pantsbuild:synthetic-test-jar:1.2.3'])