A previous CL added the auto-generated aconfig Flags.FLAG sources to
metalava's input when generating core/api/current.txt, so metalava would
expand the Flags.FLAG to their values.

Do the same for the build targets for the other (system, module-lib,
test) API signature files in core/api.

Also update the API signature files accordingly.

Bug: 301859633
Test: m checkapi
Change-Id: Id06d77e897ecfddeaa42f75cba4d6d37bee9d1b1