Stop observing a slice view when a slice is dismissed.

In order to receive updates of slices, we make sliceLiveData observes
slice views. When a slice is dismissed, it should stop observing
the view so the view can be recycled and reused.

Fixes: 120465506
Test: robotests
Change-Id: Ie4f12e9a3e6e0e7d59d3346ab17282707047bf16