fix name for Opaque data to be more clear

mesh.proto

@@ -80,7 +80,7 @@ message Data {
   enum Type {
     /// A message sent from a device outside of the mesh, in a form the mesh
     /// does not understand
-    SIGNAL_OPAQUE = 0; // NOTE: This must be 0, because it is documented in
+    OPAQUE = 0; // NOTE: This must be 0, because it is documented in
                 // IMeshService.aidl to be so
 
     /// a simple UTF-8 text message, which even the little micros in the mesh