Refine snap position behavior

- Use the stable insets to communicate the system insets to the
docked divider view.
- When calculating the sizes for the snap positions, exclude the
system insets.
- Add 3 snap position modes: 16:9 in one window, 1:1, 16:9 in the
other (phone portrait). Only 1:1 (phone landscape). Fixed relation,
1:1, 1 - fixed relation (tablet portrait/landscape).

Change-Id: If2166c5fb99f12535eeab5de18e9f5aaf433d77c