Consolidate drag to split gesture with shell transition

Refine SplitScreenTransitions with TransitionCallback to consolidate
transition handling logic.

1. Support to replace previous pairing app with drag-to-split gesture.
2. Support to switch split position by drag and drop a single-instance
   app to another side of the split.

Fix: 236814427
Fix: 236814471
Test: atest WMShellUnitTests
Test: enable shell transition, enter split screen, verified
      1. drag and drop gesture can replace apps in split
      2. drag and drop single-instance app to another side switch
	 split position
      3. quick switch and overview switch works as expected

Change-Id: Icde96369b79947e93a53ac589e64201aa32cf337