Add a reference to the snapshot when calling takeTaskSnapshot.

If takeTaskSnapshot is called multiple times for the same task during
or after a transition, the initially created snapshot can be replaced
by a later one before it is returned to the client. Adding a reference
ensures that the initial snapshot is not released before its parceling
is complete.

Flag: EXEMPT bugfix
Bug: 412389050
Test: Close the app using the gesture multiple times and verify that no
crashes occur.

Change-Id: Ie9b644bb64888f813b48e4c6de17977df8be769e