Don't perform app transition of the app is not currently visible.

Yet more special casing for the window manager...  try really hard,
if we are performing an activity transition that is behind an
opaque window (like say the lock screen or status bar) to just not
do it.  And, just as important, do a reasonable transition away from
whatever is on top.

Examples:

- If the lock screen is up, and you get a call or press the
  emergency dialer button, we fade from the lock screen to the
  new UI, instead of fading to the animation going on between
  the old and new.

- If you are in something hiding the lock screen, like the
  in-call screen, and that is hidden, then fade back to the
  lock screen.

- If you select an item from the status bar, then have the
  new item displayed behind it as the status bar rolls up
  rather than seeing a second animation.  (In fact this can't
  always be done because we may not start the transition to
  the new thing until the status bar is already going away.
  But for most cases we can do this with just one anim.)