Do not report finish in mergeAnimation...

...if the handler don't know how to process the transition.
So the Transitions can pass that transition to next handler.

Flag: NONE
Bug: 328619216
Test: manual, use alt+tab to relaunch a task where it's activity
is destroyed.

Change-Id: Idf3f78103a56e91806ec2f6e364c6e8e7f4e205c