Add api_surface prop to non-updatable droidstubs

All droidstubs created from sdk_library possess api_surface information,
while the non-updatable droidstubs do not define this property. This
change defines api_surface for non-updatable droidstubs so that the
information can be used in java_api_library

Test: m nothing
Bug: 301321012
Change-Id: I6c08003bb8c366f331440e52bbde776999fd6c94