Merge AppWindowToken into ActivityRecord

This change kept AppWindowTokenTests as it's originally based on top of
WindowTestBase, and merging that into ActivityRecordTests needs to
change the base class to ActivityTestBase, which requires additional
work and may change test behaviors. Therefore delay that to
follow-up CL.

Bug: 80414790
Test: Existing tests pass.
Change-Id: I6d73975ee77be9817c41b297f067de2d92a68499