Split preloads into media_file and data_file

Untrusted apps should only access /data/preloads/media and demo directory.

Bug: 36197686
Test: Verified retail mode.
      Checked non-privileged APK cannot access /data/preloads
Change-Id: I8e9c21ff6aba799aa31bf06893cdf60dafc04446
diff --git a/private/file_contexts b/private/file_contexts
index 03ab637..2f009ac 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -291,6 +291,8 @@
 /data/nativetest64(/.*)?	u:object_r:nativetest_data_file:s0
 /data/property(/.*)?	u:object_r:property_data_file:s0
 /data/preloads(/.*)?	u:object_r:preloads_data_file:s0
+/data/preloads/media(/.*)?	u:object_r:preloads_media_file:s0
+/data/preloads/demo(/.*)?	u:object_r:preloads_media_file:s0
 
 # Misc data
 /data/misc/adb(/.*)?            u:object_r:adb_keys_file:s0