Add basic SELinux policy for /sd-ext

* Add unionized file_contexts to extend /sd-ext support. This fixes
  /sd-ext formatting in recovery, and adds potential support for
  backup & restore of user data.
* Set SELinux to permissive mode by default. We need to add more rules
  in order for enforcing mode to work correctly.

Change-Id: I700c89640309b2d02bfcde6d0598afda6e667da9