SystemUI: Handle null keys on settings reload.

  During instrumentation or through manual command line
  invocation, null keys can be fetched from the listening
  uri's causing an IllegalArgumentException within the
  ContentProviderProxy of CMSettingsProvider's impl.

Change-Id: I4a159e838f57e6b0c32b722c72d3bb6a898f6830
TICKET: CYNGNOS-2602
(cherry picked from commit 999b3c25)