Rename demo-app to Remote Notify Demo App

[jeffallen]

• Use git mv to preserve file history when moving Kotlin files
• Change package name from org.nella.fcmapp to org.nella.RN.demo (capital RN)
• Update applicationId and namespace in build.gradle to new package name
• Rename FCMService to RnDemoService class and update all references
• Update AndroidManifest.xml with new service name and theme references
• Change app name from 'FCM App' to 'Remote Notify Demo App' in strings.xml
• Update theme name from Theme.FCMApp to Theme.RemoteNotifyDemoApp
• Replace user-facing 'FCM' references with 'notification' terminology
• Change default backend URL to https://demo.rn.nella.org
• Update README.md with new app name, package references, and default URL
• Maintain Firebase-specific technical terms where appropriate
• Update SharedPreferences name from FCMAppSettings to RnDemoSettings
• Change notification channel ID from fcm_default_channel to rn_demo_channel
• Update all UI hints and examples to use new demo.rn.nella.org domain
• Preserve all Firebase Cloud Messaging functionality and dependencies

Change-ID: s2f8322e1907a4366k