Reset mAppTransitionAnimationSpecs so they are only used once

Because we didn't reset it after sending it to window manager,
they were used again in the next transition, leading to a
wrong animation.

Also fix a bug where we waited forever because
getAppTransitionAnimationSpecs returned null.

Bug: 25583739
Change-Id: Ifcccf2d3cee649f66b4ded1d92c059acfa02ba5b