Populate baseline_file values.

Set baseline_file properties of droidstubs generated in sdk_libary.go to
incompatibilities modules auto-generated by prebuilt_api. This requires
prebuilt_api to generate the modules whether next_api_dir is set or not.

Test: TreeHugger
Bug: 180123247
Change-Id: I690604a8a54e9d9419f072a28a0c0e7734abbd85