Have you tried using a plugin and UPL ? Check my plugins:https://forums.unrealengine.com/showthread.php?127487-FREE-AdColony-AppLovin-Chartboost-UnityAds-Vungle-Sharing-OneSignal-Facebook-Everyplay&p=617273#post617273 and see how i am calling java functions.
Add the following functions to the UPL/APL FILE.
public void AndroidThunkJava_openPhotoGallery(String fileLocation){
}
public void AndroidThunkJava_openCamera(String fileLocation){
}
And then see how i call similar methods from c++.