Move PiP in response to keep clear areas changed events.

PiP window will gravitate towards bottom of the screen if it's in the bottom half of it.
PiP window will gravitate towards left/right edge of screen, whichever is closer.
PiP window will avoid occluding any reported keep clear areas.

Test: manually, existing tests pass
Bug: 183746978

Change-Id: Idb481fee7ead67734c5de0d833b281ca2a2aaaa8