Allow droidstubs to not generate any stubs

Needed to optimize the handling of the module_lib API surface which
currently has to be generated with two separate droidstubs instances,
one to generate the stubs and another to generate the .txt file. This
allows the module generating the .txt file to avoid also wasting time
generating stubs that are not used.

This change:
* Adds a generate_stubs property that defaults to true to allow the
  behavior to be customized on a per module basis.
* Uses either the stubs srcjar or the api .txt file as the OutputFile
  for the AndroidMkEntries to ensure that they get written out properly.
* Rearranges the code for generating stubs to make it easier to
  turn it off.

Bug: 146727827
Test: m droid
      Check output dir of framework-sdkextensions-api-module_libs_api
	  to make sure it does not contain any sources or srcjars.
Change-Id: Ib8025019f8a7a8cf5fa8765d76b5ad470af20006