Add check_nullability_warnings support to droidstubs.

This takes the warnings file written by metalava and diffs it against
the expected (checked-in) file, in a manner similar to the checking
and updating of the signature files. This makes it possible for a
developer adding a large API surface (e.g. by moving to a new version
of upstream sources for ojluni) to do so without being blocked on
adding the annotations, while ensuring that at all times there is an
up-to-date record of the API surface which is missing annotations.

Bug: 73448108
Test: See the other change in this topic.
Change-Id: If9ed470ef3355a0d713bb556b5561fb255e4e277