seapp_contexts

We were using system_data_file for the /data/data directories of
system UID apps to match the DAC ownership of system UID shared with
other system files.  However, we are seeing cases where files created
in these directories must be writable by other apps, and we would like
to avoid allowing write to system data files outside of these directories.
So introduce a separate system_app_data_file type and assign it.
This should also help protect against arbitrary writes by system UID
apps to other system data directories.

This resolves the following denial when cropping or taking a user photo
for secondary users:
avc:  denied  { write } for  path="/data/data/com.android.settings/cache/TakeEditUserPhoto2.jpg" dev="mmcblk0p28" ino=82120 scontext=u:r:untrusted_app:s0 tcontext=u:object_r:system_data_file:s0 tclass=file

avc:  denied  { write } for path="/data/data/com.android.settings/cache/CropEditUserPhoto.jpg" dev="mmcblk0p30" ino=602905 scontext=u:r:untrusted_app:s0 tcontext=u:object_r:system_data_file:s0 tclass=file

Bug: 14604553
Change-Id: Ifa10e3283b07f6bd6ecc16eceeb663edfd756cea
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>