Add PiP round corner on/off switch

Per discussion, enabling round corner for PiP now would force PiP into
gl composition instead of hardware one, which may cause battery drain.

Changed also in this CL:
- Infer destination bounds from TaskInfo in taskVanished
- Add a new helper class to abstract leash transactions
- Remove redundant Objects.requireNonNull

Bug: 151391111
Test: atest PinnedStackTests PipAnimationControllerTest
Change-Id: Iafa9d5ee18fa339f2e2ceb702c21eb025ede2561