Dismiss split-screen for direct enter PiP

When the PiP-able task requests to enter PiP while it's in
split-screen, for instance, from back-gesture, notify the split screen
controller to prepare an exit first.

- When it's a direct PiP request from app, like from back-gesture,
  split-screen dismisses and the other app was in split stays front
- When it's a swipe to enter PiP case, split-screen dismissed and the
  other app goes to background

Flag: com.android.wm.shell.enable_pip2
Bug: 381374430
Video: http://recall/-/aaaaaabFQoRHlzixHdtY/hTlJtDRTr7S8rCMBoOImUD
Test: Manual, back gesture to enter PiP from split, see Video
Test: atest PinnedStackTests
Change-Id: I437cce8ec430c32a3c7b1df6c334c357ddf1a8d2