Do not apply finishTransaction in DTH

Do not apply finishTransaction in DefaultTransitionHandler as the
contract should be, it's only got applied in Transitions#onFinish.

In the Split to PiP case, the left over transition is picked by
DefaultTransitionHandler and it's unaware of the actual change.
Accidentally applying the finishTransaction could cause flickers.

Flag: EXEMPT bugfix
Bug: 359667269
Bug: 312446524
Bug: 289943985
Test: Enter PiP from split-screen, make sure no flicker
Change-Id: I5fee89961421f042e9a323a8955e56cd23788040