Improve native submodule onboarding for moonlight-common-c builds

[papi-ux]

Context

Nova still relies on the native moonlight-common-c tree under app/src/main/jni. CI already checks out submodules recursively and the app pins its NDK, but local contributors can still hit opaque native build failures when the submodule is missing.

Scope

• Add a clear preflight check or Gradle error when moonlight-common-c sources are absent.
• Update contributor docs with the expected clone/submodule command and NDK version.
• Evaluate whether prebuilt native artifacts or AAR packaging are worth pursuing later, without making that a prerequisite for normal app development.

Acceptance Criteria

• A missing submodule fails with an actionable message before native compilation errors cascade.
• Local build docs match the CI setup.
• Any prebuilt-artifact plan is documented as a separate release-engineering decision, not a silent change to the current build.