Don't assign ANDROID_HOME if set already

[cccclai]

based on https://github.com/pytorch/executorch/blob/4bf7e12cce28c2a9695bb34a0028f3cf25c9b829/docs/source/using-executorch-android.md users might set up ANDROID_HOME by themselves. However the current script doesn't use that. Fix the script and have it users the one user specified

[pytorch-bot]

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❌ 1 New Failure, 17 Pending, 1 Unrelated Failure

As of commit a0a7f80 with merge base 39f474f ():

NEW FAILURE - The following job has failed:

• pull / android / build-llm-demo / linux-job (gh)
  RuntimeError: Command docker exec -t 45efe294bf69f7b8b34a8959619ff4ddb715b143a4738b46edd0b40aef2a5d0b /exec failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-llama-runner-linux (bf16, custom, linux.2xlarge, executorch-ubuntu-22.04-clang12) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128

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