Switch ART public stubs generation to use java_sdk_library

The NullFromTypeParam class needed to be marked @hide to prevent it
being used to annotate type parameters in api/public/current.txt. The
reason why that is necessary for this but not for
frameworks/base/api/current.txt is because they are generated slightly
differently.

The api/public/current.txt file is generated directly from the source
which contains @NullFromTypeParam usages.

The frameworks/base/api/current.txt file is generated from stubs which
do not contain @NullFromTypeParam usages.

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: I1bebc40e4de78eccf92b055ee24beff76511a25a
4 files changed