chore(flavors): update android to use kotlin config

[bryanoltman]

Description

New Flutter projects now use a Kotlin-based config instead of gradle. This PR updates the flavors app to reflect this and upgrades the Android gradle plugin version to 8.12.

Verified that shorebird init detects the flavors specified in android/app/build.gradle.kts

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 🧪 Tests
• 📝 Documentation
• 🗑️ Chore