Get ART public API doctag information from java_sdk_library

Uses an output tag to access the doctag_files exposed by the
art.module.public.api java_sdk_library. This ensures that the doctags
will be available from the ART prebuilts as well.

Bug: 168301990
Test: Built offline-sdk-docs with and without the change and
      diffed them. The only difference was the timestamp.js
      file.
Change-Id: Ibd51899e4aeb9b11727a54644330bbbe8b7e86a7