It doesn’t work for you because engine already overrides onActivityResult: https://github.com/EpicGames/UnrealEngine/blob/release/Engine/Build/Android/Java/src/com/epicgames/ue4/GameActivity.java#L1700
But they are good news! You can add code to the onActivityResult in MobileUtils_UPL_Android.xml!
Simply add to this file:
<gameActivityOnActivityResultAdditions>
<insert>
// code
</insert>
</gameActivityOnActivityResultAdditions>
MobileUtils plugin is designed to extending by community. What features do you want to add? Could you make Pull Request? 