Use postFrameCallback instead of a TimeAnimator to vsync PIP movements.

Per a conversation with winsonc@, we can simply post PIP resize callbacks to the vsync frame provider rather than having a TimeAnimator using the frame provider for timing. This is much simpler!

Test: atest SystemUITests
Change-Id: I6f69159ad8ccf6610609bfd3ebd3fd987e3a8c53