Add getAssetManager() method.

This method will be called from C++ side.

love/src/main/java/org/love2d/android/GameActivity.java

@@ -35,6 +35,7 @@ import android.app.AlertDialog;
 import android.content.Context;
 import android.content.DialogInterface;
 import android.content.Intent;
+import android.content.res.AssetManager;
 import android.media.AudioManager;
 import android.net.Uri;
 import android.os.Bundle;
@@ -544,6 +545,11 @@ public class GameActivity extends SDLActivity {
         return false;
     }
 
+    @Keep
+    public AssetManager getAssetManager() {
+        return getResources().getAssets();
+    }
+
     public int getAudioSMP() {
         int smp = 256;