Add extended_targets CMake hook for non-OSS JNI extensions

[psiddh]

Summary

Add a generic hook in extension/android/CMakeLists.txt that conditionally includes fb/extended_targets/CMakeLists.txt if it exists. This allows internal or partner-specific JNI extensions (e.g., Gemma3N) to plug into the Android JNI build without modifying OSS files.

When fb/extended_targets/CMakeLists.txt does not exist (as in OSS builds), the hook is a complete no-op.

Motivation

PR #17942 added Gemma3N Android demo app support, but required modifying two OSS files (extension/android/CMakeLists.txt and scripts/build_android_library.sh) with @oss-disable lines. This creates ongoing merge friction and scales poorly as more internal models are added.

This PR introduces a one-time, stable hook so that:

Build System	How it Works	Result
Internal Buck CI	Uses TARGETS files directly	No change needed
Internal CMake/Gradle	Hook includes fb/extended_targets/CMakeLists.txt which sets internal flags	Works
OSS CMake/Gradle	Hook is a no-op (fb/extended_targets/ does not exist)	Works

Test plan

• OSS builds: No behavior change. The fb/extended_targets/ directory does not exist, so the if(EXISTS ...) evaluates to false and is a no-op.
• Internal builds: Place a CMakeLists.txt in extension/android/fb/extended_targets/ to verify it gets included.

[pytorch-bot]

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❌ 7 New Failures, 5 Pending

As of commit 73fbf55 with merge base e6f7d5e ():

NEW FAILURES - The following jobs have failed:

• pull / test-coreml-bc-macos (macos-m1-stable) / macos-job (gh)
  Can't find 'action.yml', 'action.yaml' or 'Dockerfile' under '/Users/ec2-user/runner/_work/executorch/executorch/test-infra/.github/actions/check-disk-space'. Did you forget to run actions/checkout before running your local action?
• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 34ae464110786a323b55f4e0c3b2fa4294fa26014f5fe120a00e0d45d198f9bd /exec failed with exit code 1
• pull / test-samsung-quantmodels-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 677e4180c5bc0dfe6403abc7aac551a52a5a906383e1a77c574b2b3ee8345f60 /exec failed with exit code 1
• pull / unittest / macos / macos-job (gh)
  Can't find 'action.yml', 'action.yaml' or 'Dockerfile' under '/Users/ec2-user/runner/_work/executorch/executorch/test-infra/.github/actions/check-disk-space'. Did you forget to run actions/checkout before running your local action?
• pull / unittest-buck / macos / macos-job (gh)
  Can't find 'action.yml', 'action.yaml' or 'Dockerfile' under '/Users/ec2-user/runner/_work/executorch/executorch/test-infra/.github/actions/check-disk-space'. Did you forget to run actions/checkout before running your local action?
• pull / unittest-editable / linux / linux-job (gh)
  examples/models/test/test_export.py::ExportTest::test_mv3_export_to_executorch
• pull / unittest-editable / macos / macos-job (gh)
  Can't find 'action.yml', 'action.yaml' or 'Dockerfile' under '/Users/ec2-user/runner/_work/executorch/executorch/test-infra/.github/actions/check-disk-space'. Did you forget to run actions/checkout before running your local action?

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[Copilot]

Pull request overview

Adds a small, optional CMake hook in the Android JNI extension build to allow internal/partner-specific JNI targets to be integrated via a separate extended_targets/CMakeLists.txt without modifying OSS files.

Changes:

• Conditionally includes extension/android/extended_targets/CMakeLists.txt when present (no-op when absent).

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