PIP: Make resize only start from corners.

Previously it used to be possible to start resize from all edges, but we
will now only allow from the four corners, and disable the corner if
they are on the same edge as the total possible display bounds.

Bug: 156126006
Test: Drag PIP to top, try to start resize from the top corners - won't
start
Test: Draggin from the edge no longer resizes

Change-Id: Ifb0c0b500193a73c5839a335299ad6c9bc331d3c