Add property api_levels_jar_filename to droidstubs

The default is android.jar to maintain current behavior but allows users
to specify a different filepath to support generating api-versions for
docs other than the platform (e.g. auto).

Bug: 162552181
Test: m doc
Test: go test java_test
Change-Id: I9e51abaf7d5451d68ca782157d9b739f76c5da82