Update divider touch-region with DividerHandle position properly

Context : ag/11948950
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.

What changes:
After onLayout() mHandle has the final position on screen,
we reference mHandle's left or top position based on orientation
and update for corresponding touch-region

Test: atest WMShellUnitTests
Test: Enable A11y talkback, check the divider handle can be focused
Test: add log in notifySplitScreenBoundsChanged() check the split
      handle touch region dose NOT locate at screen left or top edge

Bug: 175687842
Change-Id: I7530e3e1daa373926654c3ae8bc2084566650d62