Remove setting FLAG_SLIPPERY on split screen divider

* This was allowing touches to go through to the underlying
apps when doing double tap to swap animation
* Previously we were also consuming the event if gestureDetector
consumed the touch event (but removing that works great, so now
we stop consuming the event)
* Another thing we discovered was that when doing the double tap,
the animation starts on the second ACTION_DOWN instead of the
second ACTION_UP. This appears to be default behavior for GestureDetector
which we're using to detect double taps on the view. As such, we now
explicitly run the animation on ACTION_UP.
* If we decide we want to keep/need FLAG_SLIPPERY, this bug may
also be fixed by manually listening for motion events and not using
GestureDetector, but that's a lot of work (even Gemini didn't want to do it)

Flag: EXEMPT bugfix
Test: Manual, double tapped divider with more taps during animation.
Dragged divider and did flings, seemed reasonable
Bug: 410480603

Change-Id: Ice96ae7399cc25f37a93852e5beff9a0e4e2c925