Reset CordovaBridge correctly on navigation.

If the bridge is reset incorrectly, its loadedUrl remains null, which
causes a NullPointerException when a non-file:// page tries to connect
to the bridge via prompt().

Relevant upstream code:
https://github.com/apache/cordova-android/blob/master/framework/src/org/apache/cordova/CordovaWebViewClient.java#L144

BUG=XWALK-2907
ditto.