PiP: Allow stashing via dropping on the edge.

Changes so that:
1) PiP now can be stashed when the user releases/let go of input when
PiP is currently floating on top of the left/right edge of the device.
Currently, 1/3 of the PiP needs to be off-screen in order for stash to
happen.
2) Stashing is now allowed from one edge to the other (e.g. user flings
from left to right, right to left), with VERY high velocity (>=
18000.f).

Bug: 178752503
Test: Let go of PiP when off screen, stashes
Test: fling from left to right with extremely high velocity, stashes
Change-Id: Ia0b701b75f598965d79b07b10f09118f80562f09