Update installation-android.md (#2032)

docs/docs/installation-android.md

 1. Install `react-native-navigation` latest stable version.
 
 	```sh
-	yarn add react-native-navigation@latest
+	yarn add react-native-navigation@alpha
 	```
 
 2. Add the following in `android/settings.gradle`.
 	import com.reactnativenavigation.NavigationApplication;
 
 	public class MainApplication extends NavigationApplication {
+     @Override
+	public boolean isDebug() {
+		return BuildConfig.DEBUG;
+	}
 
-		@Override
-		public boolean isDebug() {
-			// Make sure you are using BuildConfig from your own application
-			return BuildConfig.DEBUG;
-		}
-
-		protected List<ReactPackage> getPackages() {
-			// Add additional packages you require here
-			// No need to add RnnPackage and MainReactPackage
-			return Arrays.<ReactPackage>asList(
-				// eg. new VectorIconsPackage()
-			);
-		}
+	@Nullable
+	@Override
+	public List<ReactPackage> createAdditionalReactPackages() {
+		return Arrays.<ReactPackage>asList(
+			// eg. new VectorIconsPackage()
+		);
 	}
+ 	}
 	```
 
 	Make sure that `isDebug` methods is implemented.
 		android:name=".MainApplication"
 		...
 	/>
-	```
+	```