Add sdcardfs variable to storage_config_props

This property allows us to disable sdcardfs if it is present. The old
property ended up getting repurposed, so a new one was needed.
Mediaprovider will also need to access this to determine what actions it
needs to take.

Test: builds
Bug: 155222498
Change-Id: I66ac106613cbb374f54659601e4ba3f61eaecd2f
diff --git a/private/app.te b/private/app.te
index a03bcb0..9964605 100644
--- a/private/app.te
+++ b/private/app.te
@@ -35,3 +35,6 @@
     { domain -appdomain -crash_dump -rs }:process { transition };
 neverallow { appdomain -shell userdebug_or_eng(`-su') }
     { domain -appdomain }:process { dyntransition };
+
+# Don't allow regular apps access to storage configuration properties.
+neverallow { appdomain -mediaprovider_app } storage_config_prop:file no_rw_file_perms;