Generate java_api_contribution module from droidstubs module

Context
- droidstubs module are either generated from the java_sdk_library
  module or defined in the bp files.
- Since droidstubs module contains API text file property,
  java_api_contribution module can by dynamically created from
  droidstubs.

Implementation
- Add `api_surface` property in droidstubs module. This property is
  either inherited from the java_sdk_library or written in the module
  definition in the bp file.
- Add defaultable hook in droidstubs module to generate the child
  java_api_contribution module.

Test: m
Change-Id: Ica43d65614723c623cd0c155266f9844e69e5d5e