Improve error message when building Android rules without an SDK.

Fixes bazelbuild#509.

Example output:

```
$ bazel build //:all
INFO: Found 1 target...
ERROR: /usr/local/google/home/ajmichael/.cache/bazel/_bazel_ajmichael/7fcc7480abc634522e5c0cfe6b85b583/external/bazel_tools/tools/android/BUILD:236:1: Executing genrule @bazel_tools//tools/android:no_android_sdk_repository_error failed: Process exited with status 1 [sandboxed].
This build requires an Android SDK. Please add the android_sdk_repository rule to your WORKSPACE.
Use --strategy=Genrule=standalone to disable sandboxing for the failing actions.
Target //:lib failed to build
Use --verbose_failures to see the command lines of failed build steps.
INFO: Elapsed time: 0.327s, Critical Path: 0.10s
```

RELNOTES: Attempting to build an Android target without setting up android_sdk_repository will now produce a helpful error message.
PiperOrigin-RevId: 155158667

src/main/java/com/google/devtools/build/lib/bazel/rules/android/android.WORKSPACE

@@ -1,4 +1,5 @@
-bind(name = "android/sdk")
+bind(name = "android/sdk", actual = "@bazel_tools//tools/android:poison_pill_android_sdk")
+bind(name = "android/dx_jar_import", actual = "@bazel_tools//tools/android:no_android_sdk_repository_error")
 bind(name = "android/crosstool", actual = "@bazel_tools//tools/cpp:toolchain")
 bind(name = "android_sdk_for_testing", actual = "//:dummy")
 bind(name = "android_ndk_for_testing", actual = "//:dummy")

tools/android/BUILD.tools

@@ -210,3 +210,34 @@ alias(
     name = "databinding_annotation_processor",
     actual = "@android_databinding//:annotation_processor",
 )
+
+# This is the default binding of //external:android/sdk. If someone attempts to
+# build a rule that depends on //external:android/sdk without setting up
+# android_sdk_repository in their WORKSPACE, the genrule will fail with a
+# helpful error message.
+android_sdk(
+    name = "poison_pill_android_sdk",
+    proguard = ":error_message",
+    aapt = ":error_message",
+    dx = ":error_message",
+    main_dex_list_creator = ":error_message",
+    adb = ":error_message",
+    framework_aidl = ":error_message",
+    aidl = ":error_message",
+    android_jar = ":error_message.jar",
+    shrinked_android_jar = ":error_message.jar",
+    annotations_jar = ":error_message.jar",
+    main_dex_classes = ":error_message",
+    apksigner = ":error_message",
+    zipalign = ":error_message",
+    resource_extractor = "error_message",
+)
+
+genrule(
+    name = "no_android_sdk_repository_error",
+    outs = ["error_message", "error_message.jar"],
+    cmd = """echo \
+    This build requires an Android SDK. Please add the android_sdk_repository \
+    rule to your WORKSPACE. ; \
+    exit 1 """,
+)