Allow other app enter PiP when exiting current one

When there is an PiP window and another app supports PiP being present,
exiting PiP from one app should trigger the other one entering PiP.

Video: http://recall/-/aaaaaabFQoRHlzixHdtY/cSpUhLRkoyTUP73M5sXXWJ
Bug: 235877477
Test: Follow the reproduce step in bug, see video
Change-Id: Ic6c9f64885b53368b79060aa76894ea6685630c9