droidstubs: Remove unnecessary setting of DistFiles

Instead of setting DistFiles to use the api stub (if present) as the
default dist file this simply makes OutputFiles(tag) return the api
stub file for the DefaultDistTag.

Test: m nothing
      m dist sdk - before and after this change, compare result to
      make sure that there are no significant differences.
Bug: 174226317
Change-Id: I39d615c5bf8d20d59db7eeb98d942210ec0aa8ba