Provide the finish transaction when merging transitions

- Currently, the finish transaction for a transition is applied,
  followed by the start & finish transactions for all merged
  transitions.  This change provides the active handler the finish
  transaction so that it can choose to update the "final" state
  of the transition (with all merges taken into account).

Bug: 378657004
Flag: EXEMPT bugfix
Test: atest WMShellUnitTests
Change-Id: Iee54f2fd72c15c5f3b790124a51c70205b55001a