feat(ai): Add multimodal and image generation snippets

[cartland]

This commit adds several new snippets for the Firebase AI SDK, demonstrating multimodal capabilities and image generation.

New snippets include:

• Text and image input
• Text and audio input
• Text and video input
• Multi-turn chat
• Image generation from a text prompt
• Image editing with a text prompt
• Image editing in a chat session

A placeholder drawable scones.xml is also added to support the image editing snippets.

[snippet-bot]

Here is the summary of changes.

You are about to add 8 region tags.

• misc/src/main/java/com/example/snippets/ai/GenerativeAiSnippetsJava.java:69, tag firebase_ai_generative_image_model_configuration_java
• misc/src/main/java/com/example/snippets/ai/GenerativeAiSnippetsJava.java:105, tag firebase_ai_multimodal_input_java
• misc/src/main/java/com/example/snippets/ai/GenerativeAiSnippetsJava.java:128, tag firebase_ai_multimodal_audio_input_java
• misc/src/main/java/com/example/snippets/ai/GenerativeAiSnippetsJava.java:172, tag firebase_ai_multimodal_video_input_java
• misc/src/main/java/com/example/snippets/ai/GenerativeAiSnippetsJava.java:214, tag firebase_ai_multiturn_chat
• misc/src/main/java/com/example/snippets/ai/GenerativeAiSnippetsJava.java:256, tag firebase_ai_generate_image_from_text
• misc/src/main/java/com/example/snippets/ai/GenerativeAiSnippetsJava.java:286, tag firebase_ai_edit_image
• misc/src/main/java/com/example/snippets/ai/GenerativeAiSnippetsJava.java:318, tag firebase_ai_edit_image_chat

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[cartland]

Changes moved to #649