Restore system_app access to system-owned /data directories.

System UID apps want to be able to create/write to system-owned
/data directories outside of their own /data/data package directory,
such as /data/system/cache and /data/misc/keychain.  Restore access
(which was removed by Ifa10e3283b07f6bd6ecc16eceeb663edfd756cea when
system_app_data_file was introduced for the /data/data package
directories of system UID apps), but audit writes to system_data_file
so we can look at introducing separate types for these directories in
the future and ultimately remove access to the rest of the system-owned
data.

Change-Id: I573f120f23f2dd2d228aa738b31ad2cb3044ec6e
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>