Fix thumbnail header animations in freeform to recents.

The thumbnail header animations were constructed based on opening apps,
in this case Recents. This is obviously wrong, but used to work because
there was only one closing app in non-multi window world.

For the animation to work correctly, i.e. each thumbnail have its own
header animation, we need to correctly construct animations for either
opening apps or closing apps (depending on the transition type we are
seeing).

The CL also refactors handleAppTransition into smaller methods.

Bug: 24913782
Change-Id: I9f30a53d2f90f0628bb8c6238b2d0f35592b8f63