contacts: Privacy Guard support

 * If privacy guard is enabled for the calling process, return an empty
   MemoryCursor (with column names populated) instead of the real dataset.

Change-Id: I78b16f3791706636d51785e46a1f2ec656a3c9d2
Merge pull request #1 from pcarenza/jb-mr1

contacts: Privacy Guard support - You also need to fork android's calendar provider to finish up (or use CM's)