Fix artifact with LayoutTransitions on disappearing window.

Logic in performTraversals() starts a transition running at the
proper time. But when a view's parent window goes away, this transition
may not start at that time because drawing gets canceled. But the
transition still hung off of the ViewRoot, waiting until some later
drawing operation to kick it off. This resulted in some weird animations
like the Recents panel appearing and having a single item animate off of it.

The fix is to delete pending transitions when drawing is skipped.

Change-Id: I3ab7702c16e069644a163424f977350743e2cecc