Take snapshot when screen is turning off

Since we can't take a snapshot when screen is turned off, we need
to snapshot before we are turning the screen off. For this, we
- Add a callback from DisplayPowerController to give policy a
chance to do something before display will be turned off.
- Implement this callback by taking snapshots of all visible
tasks.

Test: Inspect logs/traces about screen off blocking to make sure
callback is working correctly.
Test: Insert artificial 500ms delay in onScreenTurningOff and make
sure we are unblocking screen off when turning on screen in the
meantime.
Test: Open Maps, go to recents, open maps again, scroll to another
location, toggle power button, make sure the old location isn't
shown during unlock.

Change-Id: I489f31358f838d418f894f996495946084f136a4
Fixes: 37107783