[P4] Optional :core:ai module (on-device + backend-proxied LLM)

[amaury901130]

AiAssistant interface with on-device (Gemini Nano / ML Kit GenAI) and cloud-via-own-backend implementations (never an API key in the APK), streaming UI helpers, consent screen, PII redaction, Remote Config kill switch.

Acceptance criteria

• Sample screen streams a response
• No API keys in the app

Reference: Android Base — Modernization Audit & Recommendations — section: AI-ready development