Migrate Gemini overview and Vertex AI Gemini API snippets

[kkuan2011]

Code snippets are from these existing pages:
https://developer.android.com/ai/gemini
https://developer.android.com/ai/vertex-ai-firebase

[snippet-bot]

Here is the summary of changes.

You are about to add 6 region tags.

• misc/src/main/java/com/example/snippets/ai/GeminiOverview.kt:26, tag android_gemini_ai_models_overview
• misc/src/main/java/com/example/snippets/ai/GeminiOverviewJava.java:33, tag android_gemini_ai_models_overview_java
• misc/src/main/java/com/example/snippets/ai/VertexAiGeminiApi.kt:27, tag android_snippets_vertex_ai_gemini_api_model
• misc/src/main/java/com/example/snippets/ai/VertexAiGeminiApi.kt:33, tag android_snippets_vertex_ai_generate_content
• misc/src/main/java/com/example/snippets/ai/VertexAiGeminiApiJava.java:33, tag android_snippets_vertex_ai_gemini_api_model_java
• misc/src/main/java/com/example/snippets/ai/VertexAiGeminiApiJava.java:41, tag android_snippets_vertex_ai_generate_content_java

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[ksemenova]

it would be better to made this a "suspend" function and remove "scope", but since the full code is not used in snippets we can ignore it.

[kkuan2011]

ok noted for next time, thank you Kate!