Compile android-non-updatable separately

The java stubs for android-non-updatable were previously only compiled
into the "main android stubs" target. This change refactors the build so
that we compile android-non-updatable stubs as a separate jar file. That
jar is then statically included into the main android stubs, making it
effectively a noop for the existing stub.

This has the benefit of making it possible to finalize the
android-non-updatable when the rest of the SDK artifacts are frozen,
so that we can later re-create android.jar with a mix of different
android-non-updatable and module stubs.

Bug: 184839599
Test: `m sdk dist` && diff -r out/apistubs{before,after}
Change-Id: I624e2e7ab5723eb5e27cf26dbe247aac2a799b72