Annotate ReactContext with NonNull annotation

We need handle situation where currently emitAppLaunched is being called with null context.
Related to #3392

lib/android/app/src/main/java/com/reactnativenavigation/react/NavigationReactInitializer.java

 package com.reactnativenavigation.react;
 
+import android.support.annotation.NonNull;
+
 import com.facebook.react.ReactInstanceManager;
 import com.facebook.react.bridge.ReactContext;
 import com.reactnativenavigation.NavigationActivity;
 		}
 	}
 
-	private void emitAppLaunched(ReactContext context) {
+	private void emitAppLaunched(@NonNull ReactContext context) {
 		waitingForAppLaunchEvent = false;
 		new EventEmitter(context).appLaunched();
 	}