Add GitHub workflow for building Skia on Android and macOS

[yorkie]

Adds a new GitHub Actions workflow for building Skia for Android and macOS binaries.

• Workflow Creation: Introduces a .github/workflows/build_skia.yml file to define the GitHub Actions workflow.
• Trigger Configuration: Configures the workflow to trigger on push events to the main branch and on pull requests.
• Android Build Job: Adds a job for building Skia for Android on an Ubuntu runner, utilizing Bazel and a specific toolchain configuration for Android ARM64.
• macOS Build Job: Adds a separate job for building Skia for macOS on a macOS runner, also using Bazel with a configuration and toolchain tailored for macOS x86_64.

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For more details, open the Copilot Workspace session.