Fix Android llama2 demo app after #2962 (#3032)

Summary:
This fixes the issue when the demo Android app fails to load llama2 model and returns an exit code 20.

As this failure can be captured by running the instrumentation test suite on Android devices, I also add the test spec that I'm using there for future reference.

### Testing

https://github.com/pytorch/executorch/actions/runs/8682469360/job/23808274556?pr=3032#step:12:80 loads the model successfully and shows the observed TSP now

Pull Request resolved: https://github.com/pytorch/executorch/pull/3032

Reviewed By: kirklandsign

Differential Revision: D56124177

Pulled By: huydhn

fbshipit-source-id: 7cc3987d186e670143f2ca739d29f02649091ec2