Snap to the edge when expanding the PiP window

When tapping on a collapsed PiP window, the expanded bounds should
always be in place regardless of the configured snap mode.

Cleanup also
- unused methods in PipSnapAlgorithm
- various pip snap mode support, always SNAP_MODE_EDGE going forward

Bug: 150701940
Bug: 150814251
Test: Expand PiP window
Change-Id: I3aaf0713f608b018f21e06eaf56446d7e17301b2