On some devices, when returning to the app from background, some launchers
open the SplashActivity over the NavigationActivity. In this case, simply finish the SplashActivity
and return the the preview NavigationActivity.

Fixes #1938 Closes #1900

This is a breaking change and has to be opt-in. To prevent host.clear from being called,
Override `clearHostOnActivityDestroy` in MainApplication and return false:

```java
    @Override
    public boolean clearHostOnActivityDestroy() {
        return false;
    }
```

If host isn't cleared, the next time the app is opened react context might still be initialized.
In this case we emit appLaunched event which has to be handled in Js:

```js
  Promise.resolve(Navigation.isAppLaunched())
      .then((appLaunched) => {
        if (appLaunched) {
          startApp();
        }
        new NativeEventsReceiver().appLaunched(() => {
          startApp();
        });
      })
```

Currently startApp is called from global context which results in the
app being launched when headlessJs tasks run in the background.
In order to support this use case, while not breaking existing users,
this commit adds two mechanisms which should help users detrmine if
the native Activity is running and startApp can be called safely.

1. RNN.appLaunched event is emitted is SplashActivity starts and react
   context has already been created. This is the use case of openeing
   the app while headless js task was started or has just finished and
   react context is in a "pasued" state.

2. Navigation.isAppLaunched() convenience method has been added.
   It returns a promise which when resolved, indicates if the app
   is launched and we should show the ui or not.

Usage

import {Navigation, NativeEventsReceiver} from 'react-native-navigation';

Promise.resolve(Navigation.isAppLaunched())
  .then(appLaunched => {
    if (appLaunched) {
      startApp();
    } else {
      new NativeEventsReceiver().appLaunched(startApp);
    }
  });

function startApp() {
  Navigation.startTabBasedApp({ ... });
}

This commit introduces a simple mechanism to pass deep link data from
MainActivity/SplashActivity to NavigationActivity. Note that this will
not work if the application is already running.