Guard against ContentResolver exceptions

There is a code path deep in the ContentResolver stack that will read
and then throw an exception that was written to the underlying DB by the
remote application. We don't want this to crash SystemUI, so guard
against it.

Fixes: 225359390
Test: 1. Mark Signal notification as Priority
      2. Observe no crash
Change-Id: Ie84e29bb0b9d9f19da267f2aa3ed9d8e028bbac3