Fix crash/timing issues when the `Activity` is recreated

[jayohms]

Follow up to: https://github.com/hotwired/hotwire-native-android/pull/72

Additionally, this allows an app to listen for when navigators are ready for navigation after the Activity is (re)created.

class MainActivity : HotwireActivity() {
    override  fun onNavigatorReady(navigator: Navigator) {
        // custom code for navigation
    }
}