Anything that can be dragged using the new drag manager APIs
(View#startDrag) can be dropped here; the pocket will hold
it for you until you're ready to drag it out again.
Known issues:
- currently discarding the clipping whenever you drag it
out; in the future we will only discard it once you
actually drag it somewhere
- there's a bug where dragging things back out of the
pocket's window requires an extra tap (i.e. tap once then
tap and drag)
- almost nothing uses the new drag manager yet :)
Change-Id: Iff3103e4117f31023e1e96667632d77912e21d30
524f568802