Fix flash image model and add remaining Imagen image editing snippets

[cartland]

Fixed snippets for these pages:

• https://developer.android.com/ai/gemini/developer-api
• https://developer.android.com/ai/imagen

The snippets will be added to the documentation in a separate documentation change. This is just the code.

[snippet-bot]

Here is the summary of changes.

You are about to add 10 region tags.

• misc/src/main/java/com/example/snippets/ai/ImagenSnippets.kt:106, tag android_imagen_vertex_model_configuration
• misc/src/main/java/com/example/snippets/ai/ImagenSnippets.kt:125, tag android_imagen_inpaint_insertion
• misc/src/main/java/com/example/snippets/ai/ImagenSnippets.kt:147, tag android_imagen_inpaint_removal
• misc/src/main/java/com/example/snippets/ai/ImagenSnippets.kt:172, tag android_imagen_editing_mask_editor
• misc/src/main/java/com/example/snippets/ai/ImagenSnippets.kt:262, tag android_imagen_editing_create_mask
• misc/src/main/java/com/example/snippets/ai/ImagenSnippets.kt:283, tag android_imagen_expand_image
• misc/src/main/java/com/example/snippets/ai/ImagenSnippets.kt:301, tag android_imagen_replace_background
• misc/src/main/java/com/example/snippets/ai/ImagenSnippets.kt:320, tag android_imagen_customize_subject
• misc/src/main/java/com/example/snippets/ai/ImagenSnippets.kt:348, tag android_imagen_customize_control
• misc/src/main/java/com/example/snippets/ai/ImagenSnippets.kt:372, tag android_imagen_customize_style

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