refactor(ai-logic): upgrade to Imagen 4 (#2730)

Author: thatfiredev

firebase-ai/app/src/main/java/com/google/firebase/quickstart/ai/FirebaseAISamples.kt

@@ -126,8 +126,8 @@ val FIREBASE_AI_SAMPLES = listOf(
         }
     ),
     Sample(
-        title = "Imagen 3 - image generation",
-        description = "Generate images using Imagen 3",
+        title = "Imagen 4 - image generation",
+        description = "Generate images using Imagen 4",
         navRoute = "imagen",
         categories = listOf(Category.IMAGE),
         initialPrompt = content {

firebase-ai/app/src/main/java/com/google/firebase/quickstart/ai/feature/media/imagen/ImagenViewModel.kt

@@ -88,7 +88,7 @@ class ImagenViewModel(
         imagenModel = Firebase.ai(
             backend = sample.backend
         ).imagenModel(
-            modelName = sample.modelName ?: "imagen-3.0-generate-002",
+            modelName = sample.modelName ?: "imagen-4.0-generate-001",
             generationConfig = config,
             safetySettings = settings
         )