framework: Privacy Guard + Add missing @hide annotations for 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: Iecc8df3a399e059a48829d72c8512de2669da0cd