Set security context of /adb_keys and /data/misc/adb/adb_keys.

I97b3d86a69681330bba549491a2fb39df6cf20ef introduced a separate type
for the adb_keys file.  Set the security context of the adb_keys file
accordingly by adding restorecon commands to init.rc.

Change-Id: I30e4d2a1ae223a03eadee58a883c79932fff59fe
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>