Include grpc-android and grpc-android-interop-testing in main build

[voidzcy]

Mentioned in #6132 (comment), dependencies in grpc-android and grpc-android-testing are not synced with the main build. We should maintain them using the same versions of dependencies.

[voidzcy]

For grpc-android-testing, it needs the codegen plugin as well. We would either need to check in the generated code (i.e., similar to what grpc-services is doing) or build it only when both skipCodegen and skipAndroid are false. Or leave it as is since it is special. What do you think? @ejona86

[ejona86]

Do like we do elsewhere: check in the grpc-generated code.

[voidzcy]

All done.