PiP: Polish stashing physics.

- Allow user to stash via flinging toward the edge PIP is currently
snapped to
- Bring back velocityY when user moving from stashed -> stashed (on same
edge)
- Don't allow stashing from left -> right or right -> left is already
stashed

Bug: 178881304
Test: Manual
Change-Id: Icc13998044f5870fe631487a3b4d7c123957a06a