Add a single-.so Vulkan+XNNPACK executorch JNI target

[youxie]

Summary:
Lets the executorch Android JNI sources be compiled from a Buck package other
than their own.

log.cpp and the JNI headers were not exported, and jni_helper.cpp and
log.cpp included their headers by relative path (#include "log.h"). That
form only resolves while the file is compiled inside
extension/android/jni; the moment another target picks the .cpp up as an
exported source, the include fails. This exports the missing files and
switches both includes to the full <executorch/extension/android/jni/...>
path used everywhere else in the tree.

Motivating case: building one libexecutorch.so that bundles the JNI bridge,
the runtime, and both the Vulkan and XNNPACK backends. Android's linker only
runs static initializers for libraries reached through an explicit
System.loadLibrary, so splitting the backends across several .so files
leaves their REGISTER_BACKEND globals unexecuted and the backends invisible
at runtime. Composing everything into a single .so requires reusing these
JNI sources from another package, which is what the include change enables.

No behavior change to any existing target.

Differential Revision: D118236328

cc @SS-JIA @manuelcandales @digantdesai @cbilgin

[pytorch-bot]

🔗 Helpful Links

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❌ 5 New Failures, 1 Unrelated Failure

As of commit d48ddea with merge base fdae101 ():

NEW FAILURES - The following jobs have failed:

• Cadence Build & Test / hifi-build / hifi4 (gh)
  ##[error]Refusing to check out fork pull request code from a 'pull_request_target' workflow. This workflow runs with the base repository's GITHUB_TOKEN, secrets, default-branch cache scope, and runner access. Fetching and executing a fork's code in that trusted context commonly leads to "pwn request" vulnerabilities. To opt in, review the risks at https://gh.io/securely-using-pull_request_target and set 'allow-unsafe-pr-checkout: true' on the actions/checkout step.
• Cadence Build & Test / vision-build / vision (gh)
  ##[error]Refusing to check out fork pull request code from a 'pull_request_target' workflow. This workflow runs with the base repository's GITHUB_TOKEN, secrets, default-branch cache scope, and runner access. Fetching and executing a fork's code in that trusted context commonly leads to "pwn request" vulnerabilities. To opt in, review the risks at https://gh.io/securely-using-pull_request_target and set 'allow-unsafe-pr-checkout: true' on the actions/checkout step.
• pull / unittest / linux / linux-job (gh)
  pytorch_tokenizers
• pull / unittest-buck / linux / linux-job (gh)
  pytorch_tokenizers
• pull / unittest-editable / linux / linux-job (gh)
  pytorch_tokenizers

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (matched linux rule in flaky-rules.json)
  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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[meta-codesync]

@youxie has exported this pull request. If you are a Meta employee, you can view the originating Diff in D118236328.

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