Implement immediate PiP removal

Make sure that PiP2 component handles
the case of removing PiP via means other than
direct remove from PiP menu.
These include TO_BACK cases:
e.g. when removing a multi-activity task from overview
or CLOSE cases: e.g. via
using adb shell am force stop <task>.

Bug: 335511908
Test: adb shell am force-stop <task> after entering PiP
Change-Id: Iab27c7b62bb6696eb1685137b7c1340c08e88dd0