API changes to support encryption in DPM

* New uses-policies value
* Definitions for storage domain and encryption status
* API to get and set encryption status
* Intent to launch encryption changes
* Both new calls bottom out in the DPM service and are suitable for
  a device that does not support encryption.

NOTE: Nobody should use ACTION_START_ENCRYPTION yet.  It needs a receiver
  to be built in Settings (different CL).

Change-Id: I2ae193bedbec59f6ba46c0ec7de12ecf321e5803