Migrate Intelligent Experiences (Socialite and ADK) snippets

[hamen]

Code snippets are for:

• https://developer.android.com/ai/samples/socialite
• https://developer.android.com/ai/adk

List of modifications:

• Socialite page: Migrated 2 code snippets (generative model initialization and starting a chat). Used Firebase.ai.generativeModel API.
• Adk page: Migrated 3 code snippets (agent definition, running agent, on-device models with ML Kit). Added official com.google.adk:google-adk-kotlin-core-android and google-adk-kotlin-processor dependencies to ai module. Used D23 twin pattern for on-device ML Kit initialization placeholder.

Snippets not migrated:

• Adk page: build.gradle.kts dependency configuration block because it is a Gradle configuration block.