Support packaging and extracting protos in Android aar (take 2)

[voidzcy]

Something was messed up in #440, which changed to have an ExtractIncludeProto task for each source set in Android build. However, ExtractIncludeProto tasks need to match dependency variants before pulling them in (this is configured by configurations created by the Android plugin, so the problem doesn't exist in protobuf dependencies and ExtractProto tasks). So it does not work if ExtractIncludeProto tasks are per source set basis, the build will not be able to determine the right variant of dependencies.

The original approach works. That is, getting dependencies from variant.compileClasspath for each variant. So this makes the way of extracting included protos different for Java projects and Android projects:

• Java projects extract included protos from each source set's compileProtoPath configuration (which inherits from implementation and compileOnly) as SourceSets are compilation units for Java projects.
• Android projects extract included proto from each variant's compileClasspath as variants are compilation units for Android projects.

So we will not create compileProtoPath configurations for Android source sets.

We could change Java projects to extract included protos from each SourceSet's compileClasspath with a similar approach to filter dependencies with a RESOURCES ArtifactView. With that, we can get rid of the compileProtoPath configuration, and we would no longer need the extra operation to put main sourceSet's proto into test sourceSet:

protobuf-gradle-plugin/src/main/groovy/com/google/protobuf/gradle/ProtobufPlugin.groovy

Line 394 in 5291031

inputFiles.from getSourceSets()[sourceSetOrVariantName].compileClasspath

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Majority changes in this PR is for adding test coverage for consuming protos defined in Android library dependencies.
testProjectAndroidLibrary is a standalone Android library project that provides some proto definition.
testProjectAndroidDependentBase is similar to testProjectAndroidBase but depends on testProjectAndroidLibrary and its proto definitions imports protos defined in testProjectAndroidLibrary. It covers that for all of application/unitTest/androidTest variants (#447 is created to do the same coverage for existing tests that covers Android applications consuming protos from Java projects, it's important to be covered that test variants can still import protos from dependencies of the variant-under-test).

Dependencies pulled from variant's compileClasspath has a view with "jar" attributes, so they include packaged protos.