Cleanup in StubLibraries.bp

- Merge the "full" defaults into the "base" defaults (base wasn't used)
- Reword some comments
- Fix double mention of priv_apps @SystemApi annotation in a droidstub

Test: m checkapi
Change-Id: Ide93b0f6076fec733f5a01b7fccbf280d5a8d0f6
(cherry picked from commit 1c58ffce)