Retrieve BuildConfig.DEBUG from Application

Accessing this value directly can lead to bugs as users might return a different
value from isDebug function which they override in MainApplication.

lib/android/app/src/main/java/com/reactnativenavigation/utils/ImageLoader.java

 import android.support.annotation.Nullable;
 
 import com.facebook.react.views.imagehelper.ResourceDrawableIdHelper;
-import com.reactnativenavigation.BuildConfig;
 import com.reactnativenavigation.NavigationApplication;
 
 import java.io.FileNotFoundException;
             drawable = loadFile(source);
         } else {
             drawable = loadResource(source);
-
-            if (drawable == null || BuildConfig.DEBUG) {
+            if (drawable == null || NavigationApplication.instance.isDebug()) {
                 drawable = readJsDevImage(context, source);
             }
         }