Fix doc for Android build error in xnnpack_README.md

[iseeyuan]

In scripts/build_android_library.sh, there's

ANDROID_HOME="${ANDROID_SDK:-/opt/android/sdk}" ./gradlew build

If ANDROID_SDK is not set, ANDROID_HOME will be /opt/android/sdk, which is wrong for Mac. Add export ANDROID_SDK instruction to avoid the build error if users have not exported it.

Test plan

sh scripts/build_android_library.sh

[pytorch-bot]

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❌ 2 New Failures, 1 Unrelated Failure

As of commit 045a8af with merge base 9aaea31 ():

NEW FAILURES - The following jobs have failed:

• Check Labels / Check labels (gh)
  RuntimeError: Error checking labels: PR does not have required labels
• pull / unittest / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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

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

• pull / unittest-arm-backend-with-no-fvp (test_pytest_models) / linux-job (gh) (trunk failure)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

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[iseeyuan]

@pytorchbot label "release notes: none"