Add Bazel support.

[jin]

This PR adds support for Bazel BUILD and WORKSPACE file generation, gated by the generateBazelFiles configuration option.

This has been used to generate a wide array of projects at https://github.com/jin/android-projects.

What's in this PR:

• generateBazelFiles configuration option
• Generation of a top level WORKSPACE file, with external dependency on https://github.com/bazelbuild/gmaven_rules/ to download AARs from Google Maven.
• Generation of a BUILD.bazel for each Android and Java module, with dependencies wired up. If it's a top level Android module, an android_binary target is generated. Otherwise, it's either an android_library or java_library target.

What's not in this PR:

• Testing support (this is next on the list)
• Kotlin support

[jin]

Thanks for all the comments and I'm really sorry for making such a big PR!

[NikitaKozlov]

@jin No problem. I left one more small comment, but I suggest to fix it in the PR with the language abstraction. @borisf The PR LGTM!