[PIP2] Send isTransitioningToPip callback

This is counterpart of sending PictureInPictureUiState callbacks in PiP1
when entering PiP animation starts and finishes.

Flag: com.android.wm.shell.enable_pip2_implementation
Flag: android.app.enable_pip_ui_state_callback_on_entering
Video: http://recall/-/aaaaaabFQoRHlzixHdtY/eEOFClQMApSIAuHzyokhMZ
Bug: 360195087
Test: atest WMShellUnitTests:PipUiStateChangeControllerTests
Test: atest CtsWindowManagerDeviceOther:PinnedStackTests#testEnteringPipUiStateChangeCallback
Test: Enter PiP from ApiDemos app in PiP2
Change-Id: I1bdee9554ffd6030ff15917c49f3650a224f6be2