Fix LLAMA app (#3228) (#3283)

Summary:
Pull Request resolved: #3228

Fix a UI thread issue causing crash.

Reviewed By: cccclai

Differential Revision: D56447006

fbshipit-source-id: 02eff27d4b4cd108c95b664d04679d4f92aaf5db
(cherry picked from commit 4389442)

Author: kirklandsign

examples/demo-apps/android/LlamaDemo/app/src/main/java/com/example/executorchllamademo/MainActivity.java

@@ -79,9 +79,9 @@ private void setLocalModel(String modelPath, String tokenizerPath) {
     if (loadResult != 0) {
       AlertDialog.Builder builder = new AlertDialog.Builder(this);
       builder.setTitle("Load failed: " + loadResult);
-      AlertDialog alert = builder.create();
       runOnUiThread(
           () -> {
+            AlertDialog alert = builder.create();
             alert.show();
           });
     }