[droid] Use a default WebViewActivity if no custom Activity exists

Similar to [Pull Request 7](https://github.com/calatrava/calatrava-ios/pull/7) for iOS this changes makes it so you don't need to create an empty Activity to use the shell views.

The change makes the WebViewActivity concrete, and it'll try to read a pageName from the intent that created it; the PageRegistry will now pass the pageName to the intent whenever it creates an Activity, and will default to using WebViewActivity.class whenever it can't find a
matching Activity.