Add local_aar input parameter to android-build workflow

[Copilot]

Adds support for using a custom ExecuTorch AAR file in the Android build workflow instead of the Maven dependency.

Changes

• New local_aar workflow input: Optional string parameter under workflow_dispatch accepting a URL
• Download step: When provided, downloads the AAR to llm/android/LlamaDemo/app/libs/executorch.aar
• Build configuration: Passes -PuseLocalAar=true to Gradle, triggering the existing local AAR dependency path in build.gradle.kts

Usage

Trigger via workflow dispatch with the local_aar parameter:

# Workflow will:
# 1. curl -fL <url> -> libs/executorch.aar
# 2. ./gradlew build -PuseLocalAar=true

Only applies to LlamaDemo; other matrix entries (e.g., DeepLabV3Demo) build unchanged.

Original prompt

On android-build.yml, add an input parameter for local aar, string, as url. when it's set, update useLocalAar field in https://github.com/meta-pytorch/executorch-examples/blob/main/llm/android/LlamaDemo/app/build.gradle.kts. download the url, and copy to libs/executorch.aar

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