Keep split bounds after configuration update

Before this change, split bounds always reset to half-half if
rotation. This is weird on large screen devices because they can
scale bounds in landscape too, so bounds will reset even if from
landscape to seascape.

Add restore mechanism by compute ration of current divider
and total length. Use this ratio to compute estimate position in
new configuration then get suitable snap target. Update divider
position and split bounds by this snap target postition.

Fix: 192533269
Test: pass existing tests
Test: Set density to large screen, active stage split and make top
      task larger then rotate to landscape.
Change-Id: I3ea12273348575ae968c76bdf9236155380526ec