feat(android): ability to add listeners to the Capacitor WebView

[carlpoole]

Adds the ability to register a callback to execute when certain events occur in the WebView in Capacitor. This PR includes a method to add an event when the page is finished loading. In the future this can also be used to add page error event callbacks, for instance.

Usage example: masking the view and fading it out when the web app is loaded, to hide ugly gaps in app loading

addWebViewListener(new WebViewListener() {
            @Override
            public void onPageLoaded(WebView webView) {
                fadeView.animate()
                    .alpha(0f)
                    .setDuration(duration)
                    .setListener(new AnimatorListenerAdapter() {
                        @Override
                        public void onAnimationEnd(Animator animation) {
                            fadeView.setVisibility(View.GONE);
                        }
                    });
            }});

[jcesarmobile]

This implementation only allows to add listeners from the bridge builder. I think it should allow to add (and maybe remove) listeners after the bridge has been created too, so, in example, a plugin or some custom user code in the activity class can add new listeners

[jcesarmobile]

looks good