Add /data/fonts/files directory

The updated font files will be stored to /data/fonts/files and
all application will read it for drawing text.
Thus, /data/fonts/files needs to be readable by apps and only writable
by system_server (and init).

Bug: 173517579
Test: atest CtsGraphicsTestCases
Test: Manually done
Change-Id: Ia76b109704f6214eb3f1798e8d21260343eda231
diff --git a/private/file_contexts b/private/file_contexts
index 0b21e39..89c67fc 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -551,6 +551,7 @@
 # to avoid restorecon conflicts
 /data/rollback/\d+/[^/]+/.*\.apk  u:object_r:apk_data_file:s0
 /data/rollback/\d+/[^/]+/.*\.apex u:object_r:staging_data_file:s0
+/data/fonts/files(/.*)?     u:object_r:font_data_file:s0
 
 # Misc data
 /data/misc/adb(/.*)?            u:object_r:adb_keys_file:s0