Allow bubbles to stash at the side of the screen after some time

If users haven't interacted with bubbles, stash them along the edge
of the screen to free up screen real-estate. There's a small overhang
for the bubble UI & the touch target is padded a bit so they can
still be grabbed.

The bubbles will unstash when a new bubble or update comes in.

Flag: com.android.wm.shell.enable_bubble_stashing
Bug: 341361249
Test: manual - enable flag, add a bubble, watch it stash

Change-Id: I402d4a672312a54dc6003b2e5367f20ee443370d