Android support

I would love to be able to use Bazel to create android apps with Scala. I know this isn't a common use case for Scala but at Lucid all of our Android code is Scala.

I've created a sample repo with a [`scala_android_library` rule](https://github.com/gregghz/android-scala-bazel/blob/master/rules/scala-android.bzl) (lifted mostly from the [kotlin rules](https://github.com/pubref/rules_kotlin/blob/master/kotlin/rules.bzl#L192)). Would this be something that we could get included in rules_scala?

Start with the linked sample rules/scala-android.bzl and compare its scala_android_library rule with the referenced Kotlin rules.bzl implementation. Determine what would be required to include equivalent Android support in rules_scala, then verify that Scala Android libraries can be defined and built through the supported Bazel rules.

Written by the indexing model from the issue text.