Go back to previous state when hitting back from discovery bounce

Normally when you hit back, we just close the floating view if there
is one. This makes less sense for DiscoveryBounce, since it doesn't
feel like a different state even though it's technically a floating
view. So in that case, don't consume the back press; let launcher
handle it to go to the previous state.

Bug: 80075741
Change-Id: I7270b61be70509cb2101400a12929478a5d082aa