Let TransitionHandler to clear the transition-leash

Send the remove transition-leash transaction back to the transition
handler, so it can be applied with finishTransaction together, in
case the task surface suddenly disappears due to no parents.

Bug: 213867585
Bug: 214352241
Test: atest WMShellFlickerTests:QuickSwitchBetweenTwoAppsForwardTest
Test: atest WMShellFlickerTests:MovePipDownShelfHeightChangeTest
Change-Id: I5fca942c92d7175dc2f08c3f14f4da464405709c