[Android] Update docs

[kirklandsign]

You no longer need export FLATC_EXECUTABLE

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/3286

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❌ 3 New Failures

As of commit 6e01779 with merge base d3326a2 ():

NEW FAILURES - The following jobs have failed:

• Android / test-demo-android (buck2) / linux-job (gh)
  RuntimeError: Command docker exec -t 5bd2ee14fcccf373694879206a82a7659c6257418d9e6a3d8cb499db14aecacf /exec failed with exit code 1
• Apple / test-demo-ios / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 74
• Apple / upload-frameworks-ios (gh)
  Credentials could not be loaded, please check your action inputs: Could not load credentials from any providers

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