Prepare to generate "runtime" stubs in droidstubs

In order to prepare generating "runtime" stubs in droidstubs, this
change performs some cleanups, which are:

- Modify droidstubs providers methods to take a StubsType enum as an
  input, in order to remove exportable-specific providers. The methods
  are also modifed to return an error when the artifact is not provided
  for the input StubsType.

- Wrap droidstubs artifact to a struct. This removes artifacts common to
  all stubs types from being written out as a individual module property
  (e.g. exportableAnnotationsZip -> exportableArtifacts.annotationsZip)

Test: m nothing --no-skip-soong-tests
Bug: 319162970
Change-Id: I6d6c57c5417ca71eab5ddda7981b71fd84fa6ee1