Fix ATen/Config.h missing on Android after arvr_mode constraint migration

[mvsfb]

Summary:
Recently D99903392 broke our builds (see multisect on that diff), this is an attempt to fix this.

D99903392 converted arvr_mode from a constraint_setting to a
constraint() rule. This changed how the select() in
aten_headers_for_executorch resolves: Android platforms now match
arvr_mode[enabled], routing to ovrsource_aten_Config.h. But that
target is an oxx_static_library that produces no outputs on Android

Fix: use a nested select to fall back to generated_aten_config_header
on Android, while preserving the OVR variant for non-Android ARVR
platforms. Same pattern as D100438873 but for Android.

Differential Revision: D100832205

[pytorch-bot]

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❌ 1 New Failure, 4 Unrelated Failures

As of commit cb77510 with merge base eaef2ed ():

NEW FAILURE - The following job has failed:

• pull / unittest-editable / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / test-models-linux-basic (vit, xnnpack-quantization-delegation, cmake, linux.2xlarge, executorch-u... / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-sqnr-static-llm-qnn-linux (smollm2_135m) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / unittest / windows / windows-job (gh) (matched win rule in flaky-rules.json)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (matched win rule in flaky-rules.json)
  ##[error]The operation was canceled.

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[meta-codesync]

@mvsfb has exported this pull request. If you are a Meta employee, you can view the originating Diff in D100832205.

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