Add Kotlin Android support

[dsyang]

Vanilla kotlin library support was added in #810. the next big step would be to add android support.

From @bkase's comment on #810 (comment) it looks like the problem is we need the equivalent of android_library() for kotlin. I think that could be pretty straight-forward.

We would need create a new AndroidKotlinLibraryDescription that behaves almost exactly like AndroidLibraryDescription excepts it compiles with kotlinc instead of javac.

A quick look through the source code suggests we'd need to do the following:

• Refactor AndroidLibrary to take in a BaseCompileToJarStepFactory (it currently defaults to javac)
• Copy AndroidLibraryDescription into a AndroidKotlinLibraryDescription. Basically doing the same thing except passing in KotlincToJarStepFactory to the AndroidLibrary
• Refactor the common stuff out of the two android library descriptions.

I'm not sure when I'll have time to tackle this yet. If someone else wants to take a shot go for it!

[bolinfest]

@bkase Want to sign up for this one?

[bkase]

Very busy through September, but afterwards I may have some spare cycles

[kageiit]

Does this account for kapt support as well?

[zbsz]

Got something similar in #913, but instead of creating specific Android{LANG}LibraryDescription I opted to use language config parameter. So far have it implemented for Scala, adding Kotlin and other jvm languages should be quite straightforward then, just wanted to get it accepted for Scala first.

[dsyang]

#937 enables this. @kageiit I don't think so, but I think it'd be best to open up a new issue for that