safe FirebaseInstanceId.getInstance() access.

`FirebaseInstanceId.getInstance()` can throw exceptions. For example if Firebase was not initialized.

Fix sound type definition

Delete instanceId function

Documentation update

Support Android activity aliases

Fix type Listner to Listener

Just return the className. The old code didn't work with activity aliases. For example, when launching with `<activity-alias android:name=".SplashActivity" android:targetActivity=".MainActivity" />`, the old code would try to get the class for `SplashActivity` which threw `ClassNotFoundException`.

`android/settings.gradle` is the correct path for React Native v0.45.1

Thank you for the nice library.
Fixed typo to sample source code.

Update README.md

Fix call to setApplicationIconBadgeNumber