Fix incorrect bounds of split-screen when rotation changes

Before calculating docked bounds, the rotated bounds should not
be set again by previous bounds.

Also simplify the condition of the bounds calculation a bit.

Bug: 132188186
Test: atest SplitScreenTests
Test: manual - Put 2 apps in split-screen mode, rotate device
      to landscape and then portrait. The divider should be
      be at the middle.

Change-Id: I0cb30093363ea5ca499bd1c2a66c29deffcd21f1