android-interop-test isn't compiled in CI for PRs

[ejona86]

#4853 broke android-interop-test:
https://source.cloud.google.com/results/invocations/8efa4293-a339-497f-a963-5951df1a5a4b/targets

However, if you look, the CI is completely green for it. It appears this is because android-interop-test is not compiled as part of android.sh, so it is only once things get merged to master we notice the breakage.

As another note, we discovered (and should remember for the future) that gradle build didn't notice the compilation failure. Apparently build does not include the test app.