This CL adds a new flag to tell metalava that it's invoked while
*only* checking compatibility. In that case it will ignore various
other flags being passed in to it (such as --stubs, which it's
currently unconditionally passed but is not useful and is costly
during compatibility checks.)  It also renames the --update-api
flag (meant for similar purpose during make update-api, where
we don't need to generate stubs) to be called --only-update-api
to make its purpose clearer.

Test: Unit test included
Change-Id: I3f105746c5f76356b8dca0f1709ee2a4413ff938