Remove legacy involveSplitScreen API

The onSplitScreenInvoked was used to notify system ui to reset the
divider position before entering split screen. With
SplitScreenTaskOrganizer, WM Shell library could know the exact time
when entering/leaving split screen mode. Removes the legacy API and
resets divider position when leaving split screen mode.

Fix: 168869136
Test: atest WMShellTests
Test: atest SystemUITests
Test: manual check recents and split screen behavior
Change-Id: Id14925619d1ec7c014a3e306fd99ec3734f227da