Remove api_files property from java_api_library

java_api_contribution provides api_surface information, but files
directly passed to java_api_library do not possess such information.
Currently, the api surface is assumed via naming convention for api
files passed via api_files property, but this is fragile.

This change removes the api_files property from java_api_library and
enforce all api files to be passed via java_api_contribution

Test: m nothing --build-from-text-stub
Bug: 300964421
Change-Id: If01d9ed978fe469d4ee0d685582a51629ebecc56