Fix flickers when changing control

- Do not reset system bar visibility when changing control target
- Do not apply hide transaction when gaining control, because that
may result in a single-frame flicker because it will conflict with
the animation
- Check requestedVisible instead of InsetsState.isVisible in
DecorView to avoid drawing the bar backgrounds transiently
- Abort transient mode when focused win changes.

Bug: 150195782
Bug: 152014877
Change-Id: I8bc9cdc89ce7364984ade8146e12a706ad5e8edb