Use ART stubs source generated by java_sdk_library

Bug: 168301990
Test: Built offline-sdk-docs with and without the change and
      diffed them. There were some changes other than the timestamp.js
      file. With these changes classes that implemented the
      Object.equals(Object) method had some extra documentation that
      explained that the parameter could be null. The reason for this
      change is that the stubs generated with this change includes
      android.annotation.Nullable whereas previously they included
      androidx.annotation.Nullable. The change is a very minor
      improvement over what is currently uploaded to
      developer.android.com/reference so not worth backporting.
      Diffed api/public/current.txt against
      frameworks/base/api/current.txt and made sure that the latter was
      a superset of the former and there were no differences in the area
      that overlapped.
Change-Id: I3767948b4659b18b813c8eed66258af98ac4a241