Update location of policy files

The location of policy files has changed from
/data/system to /data/security.

Also, a name change to mmac enforcing property
name to place it under persist.mmac. namesepace.

Adds api's for getting and setting the MMAC
enforcing mode.

Change-Id: Id07f3932e9595432a7f3f58c9a41e7588bad47de