Collapse the stack on clicks rather than on down

Fixes an issue where you'd collapse the bubble
accidentally when performing back gesture in the padding
around the expanded view.

This means things will hide 'on up' rather than 'on down'
but I think that's fine / more expected behavior.

Removed some old code.

Test: manual - expand the bubble stack, open a 2nd activity in the
               bubble, perform gesture back => 2nd activity goes
               away & the bubble doesn't collapse
Bug: 190102948
Change-Id: I016bd9b3091938974f3330f4d005d4ef6bcccfee