Reduce getting thread statically

- So the threads won't be bring back right after tear down
  and execute something unexpectedly.
- Add annotation to make AnimatingActivityRegistryTest thread safe.
- Remove the dependency of test base from
  PendingRemoteAnimationRegistryTest that reduces 90% execution time.

Bug: 144611135
Test: atest AnimatingActivityRegistryTest
            PendingRemoteAnimationRegistryTest

Change-Id: I281c475a6f6ae9ab4fb99b2f8cad200688774f21