android install docs: change MainActivity.java to MainApplication.java
android install docs: change MainActivity.java to MainApplication.java

In recent versions the `getPackages` method is implemented in MainApplication.java
bsudekum/react-native-mapbox-gl,bsudekum/react-native-mapbox-gl,bsudekum/react-native-mapbox-gl,bsudekum/react-native-mapbox-gl