Android: allow building Vulkan/XNNPACK backends as separate shared libraries

[msluszniak]

Summary

The Android AAR links every enabled backend into libexecutorch_jni.so, so an
app pays for all of them even when it uses one. Vulkan and XNNPACK are the
largest contributors, and an app shipping only the CPU path still carries the
GPU backend.

This PR adds EXECUTORCH_BUILD_VULKAN_BACKEND_SHARED and
EXECUTORCH_BUILD_XNNPACK_BACKEND_SHARED, both defaulting to OFF, so
existing builds are byte-for-byte unchanged. When set, the backend is emitted as
its own .so that whole-archives the corresponding static archive, and the JNI
library no longer links it — consumers load only what they need.

Two details worth calling out for review:

• extension/android/jni/jni_noop.cpp is an empty translation unit. A SHARED
  target whose only content is a whole-archived static library still needs one
  source file for CMake to invoke the linker on.
• custom_ops links xnnpack_backend directly, which would pull the backend
  back into the JNI library and defeat the split, so that link is dropped when
  the shared XNNPACK build is selected.

build_android_library.sh forwards both variables so they can be driven from
the environment.

Test plan

• Default build (both flags OFF) produces the same single libexecutorch.so as
  before.
• With EXECUTORCH_BUILD_VULKAN=ON EXECUTORCH_BUILD_VULKAN_BACKEND_SHARED=ON,
  the Vulkan backend builds as a separate .so and an app loading it runs
  Vulkan-delegated models on device (Galaxy S26 Ultra, Adreno).
• This split is what react-native-executorch ships on Android today, so it has
  had real-world exercise across both backends.

cc @SS-JIA @manuelcandales @digantdesai @cbilgin

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/21849

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⚠️ 12 Awaiting Approval

As of commit 8167cb1 with merge base 33f68d1 ():

AWAITING APPROVAL - The following workflows need approval before CI can run:

• Build documentation (gh)
• Build Presets (gh)
• Lint (gh)
• periodic (gh)
• pull (gh)
• Test ARM Backend (gh)
• Test CoreML Backend (gh)
• Test QNN Backend (gh)
• Test QNN Windows MSVC build (gh)
• Test Vulkan Backend (gh)
• Test WebGPU Backend (gh)
• Test XNNPACK Backend (gh)

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