Add SDL_GetDisplayDPI implementation on Android. (thanks Rachel!)

android-project/app/src/main/java/org/libsdl/app/SDLActivity.java

@@ -18,6 +18,7 @@ import android.widget.Button;
 import android.widget.LinearLayout;
 import android.widget.TextView;
 import android.os.*;
+import android.util.DisplayMetrics;
 import android.util.Log;
 import android.util.SparseArray;
 import android.graphics.*;
@@ -613,6 +614,10 @@ public class SDLActivity extends Activity {
         return SDL.getContext();
     }
 
+    public static DisplayMetrics getDisplayDPI() {
+        return getContext().getResources().getDisplayMetrics();
+    }
+
     /**
      * This method is called by SDL using JNI.
      */