Add wpa neverallow rule

wpa should never trust any data coming from the sdcard. Add a
compile time assertion to make sure no rules are ever added
allowing this access.

Change-Id: I5f50a8242aa30f6cc0cfd89d82b2b153625105f6

wpa.te

@@ -37,3 +37,11 @@ allow wpa keystore:keystore_key {
 userdebug_or_eng(`
   unix_socket_send(wpa, wpa, su)
 ')
+
+###
+### neverallow rules
+###
+
+# wpa_supplicant should not trust any data from sdcards
+neverallow wpa sdcard_type:dir ~getattr;
+neverallow wpa sdcard_type:file *;