Add XOR example to Android JNI setup

[georgehong]

Summary

In preparation for adding the Android JNI, add the xor training model to the test prep script. Since the script will be adding the .ptd external weights to the test directory, also update the git ignore to account for this.

• xor_only.pte -- includes model weights
• xor.pte and xor.ptd includes XOR model with external weights, to be used together.

Test plan

Ensure that xor_only.pte, xor.pte, and xor.ptd are generated. Check CI is successful.

sh executorch_android/android_test_setup.sh

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/12250

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❌ 1 Cancelled Job, 2 Unrelated Failures

As of commit d55c131 with merge base 557b4de ():

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / android / build-llm-demo / linux-job (gh)
  ##[error]The operation was canceled.

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-models-linux (mobilebert, portable, linux.2xlarge) / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / test-models-linux (mobilebert, xnnpack-quantization-delegation, linux.2xlarge) / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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