Update the divider touch-region properly

The divider view itself isn't really relayouted (because now
it's surface just gets moved around). This means we actually
need to update its touch region when moving the surface instead
of in relayout. Additionally, because the views themselves
don't move, update the touch-region from divider position
instead.

Bug: 159304463
Test: dumpsys uiautomator --windows or turn-on talkback and
      dumpsys accessibility. Either way see that divider window
      region/bounds is reported correctly.
Change-Id: Ic531ac44af772d1ae187a1087d5fe3b23d21b96e