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 now, so removes the API and
deals with the divider bar position in WM Shell library scope directly.

Resets mRatioPositionBeforeMinimized when dismission split screen since
the informaiton is no longer needed, it will leverage the creation flow
of DividerVew to assign the default target to the new divider bar.

This change will also make sure to reset the divider bar target even
when users are not triggering split screen from recents app shortcut.

Fix: 168869136
Test: atest WMShellUnitTests
Test: atest SystemUITests
Test: manual check the split screen behavior
Change-Id: Icd84a8c1559d0f5f0f99b22445410112fa517efe