Define contextmount_type attribute and add it to oemfs.

Several device-specific policy changes with the same Change-Id
also add this attribute to device-specific types.

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>

(cherry picked from commit d2503ba8)

Change-Id: I08a718ba0d91641de720440e16abc0a04f5ec5a5