Send isTransitioningToPip when pip anim starts

This is to fix a regression from ag/28340648, which breaks the contract
that PictureInPictureUiState(isTransitioningToPip=true) should be sent
to the app at the very beginning of the enterign PiP animation.

In this change
- We explictly send the callback when SysUI is notified by the Launcher
  that the animation is started
- The path to trigger the callback for button navigation mode is kept
- A check for swipe-pip-to-home is added before sending the callback, to
  make sure there is no double callback when auto-enter-pip

Flag: EXEMPT bugfix
Video: http://recall/-/aaaaaabFQoRHlzixHdtY/cKlb6lqM1DHGk2hG4eaBYB
Video: http://recall/-/aaaaaabFQoRHlzixHdtY/dzrznErP2hQFLs7cajPFKx
Bug: 359264914
Test: Verify callback is sent at the beginning in both gesture and
      button navigation mode
Test: atest CtsWindowManagerDeviceOther:PinnedStackTests
Change-Id: Ibbfb78bb7b410b88587595be835f8d003c0bd619