Fix multi-user silent notifications on lockscreen

Read the current user's setting for showing silent notifications
on the lockscreen whenever user switching occurs. Otherwise, when
switching to another user, the former user's setting will be in
effect for the new user, which may cause silent notifications to
be unexpectedly hidden or visible on the lockscreen.

Change-Id: I65b10f82307cf8eecaafcd1a76ddb5f98bd7c326