Fix onNewIntent

I had trouble using multiple functions of the plugin together: 

```javascript 
//handle activities if app is already running
window.plugins.webintent.onNewIntent(function(url) {
    if(url !== "") {
        console.log("[INTENT] onNewIntent" +  url);
    }
});

//handle app invoke via activity
window.plugins.webintent.getUri(function(invokeUrl) {
    console.log("[INTENT] handleAndroidOpen: " +  invokeUrl);
});
``` 

The problem was that i did not get the `onNewIntent` events, but they appeared in the console as `Attempted to send a second callback for ID:...` which happens if the callback has already been called before [Source](https://github.com/apache/cordova-android/blob/master/framework/src/org/apache/cordova/CallbackContext.java#L57) 

The actual problem is that the callback get's saved in the execute function [Source](https://github.com/Initsogar/cordova-webintent/blob/master/src/android/WebIntent.java#L40) which happens on any execute, not just on `onNewIntent`. 

So i'm saving the reference only on `onNewIntent` executes and renamed it, to make clear what this reference is good for. 

I hope you merge this request as the `onNewIntent` method is quite unusable at the current state.