Fix demo for Android

[javiercr]

The sample demo was failing in Android with this error:

~/react-native-fbsdk/sample/HelloFacebook/android/app/src/main/java/com/hellofacebook/MainApplication.java:59: error: getUseDeveloperSupport() in <anonymous com.hellofacebook.MainApplication$1> cannot override getUseDeveloperSupport() in ReactNativeHost
    protected boolean getUseDeveloperSupport() {
                      ^
  attempting to assign weaker access privileges; was public

That method was changed from protected to public here.

This PR fixes it :)

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[dzhuowen]

Thanks for the PR!