Set is_stubs_module for stubs modules

This change sets is_stubs_module property to true for combined modules
genreated in `combined_apis` and non-updatable stub modules, which are
not generated via java_sdk_library but are maintained separately.

Test: m nothing --no-skip-soong-tests
Bug: 310504781
Change-Id: I1293208e575a76db2cc4dc6c8564369804bb4ac5