framework: Privacy Guard

 * Introduce a new privacy feature which allows the user to run an
   application with reduced visibility into his or her personal data.
 * Adds a per-application flag and simple API to determine if this flag
   is enabled for the current or calling process.
 * This flag can be used by content providers to decide if they should
   return a limited/empty dataset.

Change-Id: Id7c54d728e63acb2b02a2a9322930b54949f6c5d