Akring integration tests fix

Summary:
Fix a bug that allows us to run integration tests in our android project, where RN is specified as a module to our project.

sdkHandler does not exist. The android documentation suggests that we should be using android.ndkDirectory instead.

http://tools.android.com/tech-docs/new-build-system/migrating-to-1-0-0

An alternative solution would be to set the environment variable ANDROID_NDK, but we do not want to rely on setting this environemnt variable at a system wide level.

We ran two tests.
1.) perform a gradle sync from within our Android studio project, and verify there is no error output.
2.) build android using ./gradleW, from command land. Verify there are no build errors.
Closes facebook#14136

Differential Revision: D5327421

Pulled By: shergin

fbshipit-source-id: d9e18519a8ca318f2026eb409b90cb09e2adbda1

ReactAndroid/build.gradle

@@ -168,7 +168,9 @@ def findNdkBuildFullPath() {
         return new File(ndkDir, getNdkBuildName()).getAbsolutePath()
     }
     def ndkDir = android.hasProperty('plugin') ? android.plugin.ndkFolder :
-            plugins.getPlugin('com.android.library').sdkHandler.getNdkFolder()
+            plugins.getPlugin('com.android.library').hasProperty('sdkHandler') ?
+                    plugins.getPlugin('com.android.library').sdkHandler.getNdkFolder() :
+                    android.ndkDirectory.absolutePath
     if (ndkDir) {
         return new File(ndkDir, getNdkBuildName()).getAbsolutePath()
     }