Create java_api_contribution and java_api_library module

Context
- Droidstubs module is currently responsible not only for java api stubs
  generation, but also for checking api equality and compatibility.
- Generating stubs with incomplete api text file that does not list
  entire api surface is done through metalava implicitly adding unlisted
  methods from java source files to the stubs in droidstubs module.
- These factors make java stubs generation harder to debug, thus
  introduce `java_api_contribution` and `java_api_library` modules to make java
  api stubs and jar generation more explicit in Android.bp level and
  eventually easier to debug.

Implementation
- `java_api_contribution` module is included in api domains' directory and
  lists api text file directory to be added to the api surface
- `java_api_library` collects all api text file that forms the api surface
  and creates stubs invoking metalava. Generated java stub files are
  converted into `.srcjar`, and eventually `.jar` file which is the complete
  api surface.

Test: m
Change-Id: I86f097cc8592334a5eaa900cec12764c5fcc09e7