Clean up ActivityView workaround for release

After ag/6377578 it is now safe to call release() on ActivityView
and detach it from the window without waiting. This removes the
hack which adds it to a temporary view while it is released.

Test: manually added and rapidly removed many bubbles, no crash
Change-Id: I21497c30e518c257a70fe494ecaf0a6f1af8ad21