Quickswitch with staged split

* UI polish/animations needed.
* One known bug (b/198310766), temp work around is to
swipe up to home.

Bug: 181704764
Test:
* Open apps in staged split and quickswitch
between GroupedTaskView and fullscreen apps.
* QS to fullscreen app and then go into overview
and re-launch split screen tasks
* QS to fullscreen app, wait 5 seconds,
swipe into overview, no GroupedTaskView shown

Change-Id: I0ce10a944d86be5c927eeaaef922559a40f39923