Add an isSecretEstablished() getter to CordovaBridge

Not being used, but might be of use to an Engine plugin or a Whitelist
plugin.

framework/src/org/apache/cordova/CordovaBridge.java

@@ -107,6 +107,10 @@ public class CordovaBridge {
         expectedBridgeSecret = -1;
     }
 
+    public boolean isSecretEstablished() {
+        return expectedBridgeSecret != -1;
+    }
+
     /** Called by cordova.js to initialize the bridge. */
     int generateBridgeSecret() {
         SecureRandom randGen = new SecureRandom();