Use the proper transaction when finishing a split screen transition

It will restore the shifted split layout after entered overview panel,
this makes sure to update the proper finish transaction so the split
layout will be updated properly after animated the transition.

Also support to modify the finish transaction when a transition got
merged, so we can append clean-up operations for the cases like merging
an enter split screen transition to another handler.

Fix: 238856352
Test: enabled shell transition, verified split layout has been
      shifted/restored properly after re-entering a split pair.

Change-Id: I630c54ed7c25246634c314eb9b2561a8556d8570