Entry animation for docking windows.

We achieve the animation in the same way we would do that for maximizing
windows. We preserve the exiting window of relaunched activity until the
activity adds a new one. Then we animation the new window from the
bounds of the old window to the dock bounds.

This mostly reuses existing infrastructure for maximize animations, but
unfortunately needs scaling. The before window might be have one
dimension larger than after window and using cropping is not sufficient.

Change-Id: I9538ba983a0af1e64ea48aad6836173d6fd25f3b