Replace references to droiddoc with droidstubs

The java_sdk_library code used to create droiddoc and now it creates
droidstubs but it still referenced droiddoc internally. This change
removed all the remaining references except the ones that are visible
externally.

Bug: 153443117
Test: m nothing
Change-Id: Ie883eb4590f9c091d4149d7b17d7d3d91b1b5c6a