Support passing multiple last api files to metalava in droidstubs

This change supports passing multiple last api files to metalava,
instead of emitting an error when multiple files are passed as inputs.

The last api file providing modules are filegroups or genrule modules
that output exactly one output file. This change modifies that behavior
so that the filegroups can output multiple api files.

The last api file providing modules are (mostly) generated from the
`prebuilt_apis` sdk module. However, these modules only provide a single
api scope txt files (e.g. system api scope last api file filegroup
would only provide last system api file, not system + public). This
change modifies `prebuilt_apis` by generating "combine" filegroup
modules, that include api files of the subset api scopes as well.

The ordering of the files are handled when generating the combined api
filegroup modules, and droidstubs module does not check whether the
passed api files are in order from the narrowest api scope to the widest
api scope.

Submission of this change will be done once metalava fully supports
handling multiple api files input for api lint and nullness migration.

Test: m nothing
Bug: 321827591
Change-Id: I2066e7ceb7ee7c6d0fd87cd43bfd08db906d4b8f