settings: Implement SummaryProvider for external tiles

 * This is left as a TODO in the source, and is needed functionality
   for the dashboard to be consistent when many remote components
   are registered.
 * The remote component should list the class name of a broadcast
   receiver which will handle the REFRESH_SUMMARY action in it's
   activity metadata. An optional key to be sent in the broadcast
   can also be specified (to identify the component if the receiver
   serves many settings). The receiver should set the "summary"
   entry in the result extras, which are then returned back to us.

Change-Id: I4f6d43fd471d1047871ddeac85101888d0145701