Allow apps to read preloaded photos

(cherry picked from commit e01654f9)

For Retail Demo mode, we need to preload photos in
/data/preloads and allow regular apps to access the
photos returned by the media provider from the preloads
directory.

Bug: 29940807
Change-Id: Ic1061dac55ace1b125ae04b5b0c70aae9aa0c732

untrusted_app.te

@@ -94,6 +94,10 @@ allow untrusted_app self:process ptrace;
 allow untrusted_app sysfs_hwrandom:dir search;
 allow untrusted_app sysfs_hwrandom:file r_file_perms;
 
+# Allow apps to view preloaded content
+allow untrusted_app preloads_data_file:dir r_dir_perms;
+allow untrusted_app preloads_data_file:file r_file_perms;
+
 ###
 ### neverallow rules
 ###