Bug 1435079 - Stop running most Android non-gradle tests; r=jmaher

Android non-gradle tests run against an Android apk built without
gradle, "Bng" on treeherder; confusingly, this platform is labelled
"Android API16+ Gradle opt". This patch significantly reduces the
tests run on this platform: now only robocop tests are run.

The full set of Android tests, including mochitests and reftests,
continue to run on the "Android 4.3 API16+ opt" platform.

taskcluster/ci/test/test-platforms.yml

@@ -297,8 +297,9 @@ android-4.2-x86/opt:
     test-sets:
         - android-x86-tests
 
+# android-4.3-arm7-api-16-gradle/opt actually uses the non-gradle build
+# which is tier 2, and requires only a smoketest, like robocop
 android-4.3-arm7-api-16-gradle/opt:
     build-platform: android-api-16-gradle/opt
     test-sets:
-        - android-common-tests
         - android-opt-tests