tools

Until now, metalava would look to the command line flags
to determine how to interpret types in signature files
that it read. Now it instead makes that decision based
on the format header. Also some other cleanup, such as
getting rid of the now obsolete staged nullability support
concept.

Test: Unit tests updated
Change-Id: I6c14f14410fc6ec1c4c271fdd51ac26f87124a9a