Waiting for reinflating the notifications until the user switched

Previously we would react to density changes immediately even if
we were still switching users, which could apply a new density
to the wrong notifications.
We're now waiting until we actually switched.

Test: add notifications on different user with different density, switch
Change-Id: Ieb0094ba6740bd5b9c53b275127bd128bc19236e
Fixes: 30839163