Add an api_files property in java_api_library

java_api_contribution is useful to java_api_library when the api files
are not colocated.

If they are colocated (e.g. in Multi-tree assembled api_surfaces
directory), it is useful to refer to them directly without nedding to
create a java_api_contribution module.

Test: In build/soong, go test ./java
Change-Id: I5b4e557068a1e5c71a80c76452030e72ec83a696