Add api_surface prop to api surface contributing droidstubs

All droidstubs created from sdk_library possess api_surface information,
while "art.module.toolchain.api" do not define this property. This
change defines the api_surface property for the module so that the
information can be used in the java_api_library.

Test: m nothing
Bug: 301321012
Change-Id: I9cbbfec8a42eee5a0b668c4b7be306b24f4a52f4
1 file changed