Settings: Listen to preferapn changes via observer.

  If a user were to go into the apn settings panel before
  onDataSetupComplete was able to write a preferred apn
  setting, the user would be met with a full panel of apns
  but no selected entry until a callback for a data state
  change came in. So instead of just depending on broadcasts
  from the framework, utilize a content observer to to repopulate
  the list on change.

TICKET: PAELLA-53
Change-Id: Ia89a43a9ff7dfc708a57502712fff8d30817eb21