Create HandlerThread for TaskFragmentAnimationRunner

TaskFragmentAnimationRunner is initialized using the
looper from the caller thread. This may cause a NPE
because the looper may be null if the
TaskFragmentAnimationRunner is initialized from a
non-main thread.

This CL creates a HandlerThread for the
TaskFragmentAnimationRunner so that it can be
initialized from any thread.

Bug: b/207662199
Test: atest CtsWindowManagerJetpackTestCases:ActivityEmbeddingLaunchTests
Change-Id: Id06b03380dc6dfa9122fca293df8948b4374c966