package:android_jni package:java_jni repositories

> https://github.com/mahesh-hegde/android_jni proof of concept using current PR dart-lang/jnigen#110

_Originally posted by @mahesh-hegde in https://github.com/dart-lang/native/issues/731

If we want to start making the generated bindings available (as very experimental) we need to solve some logistics questions.

Should `package:android_jni` and `package:java_jni` live in this repository? Or in separate ones.

Arguments for the same repository:

* Whenever we change the generated code, we would atomically change the generated bindings as well in the same commit.
* All tests of the standard library packages would be run on every jnigen commit.

Arguments for a separate repositories:

* We probably want to write a lot of tests and examples using the generated bindings, these commits would all show up in the same git history.
* Flexibility to not depend on the latest version of package:jnigen/package:jni.
* We could with some CI trickery probably run all the package:android_jni/package:java_jni tests with newly generated bindings with jnigen and a path dependency to the last version of jnigen/jni on the jnigen/jni repository.

package:java_jni and package:android_jni need to be different packages, because using the Android APIs on a desktop/CLI app that spawns a JVM itself, there are no Android APIs.

FYI @mit-mit