Add aconfig: enable_pip_ui_state_on_entering

This CL adds a new flag into multitask.aconfig, when it's enabled, there
will be an extra callback to Activity#onPictureInPictureUiStateChanged
when system is putting the Activity into PiP mode.

Bug: 303718131
Flag: ENABLE_PIP_UI_STATE_ON_ENTERING
Test: N/A
Change-Id: I4997accbf58208ec50b26b21ecd0fef996b5813a