iOS 9 support dropped

This change-breaking release focus on iOS

• On install Remote Notification is set as a Background Mode automatically;
• Not only copies the Google Services configuration file on build, but also on install;
• Firebase now is handled manually, due to complications of auto-swizzling;
• Firebase dependencies are now upgraded to 6.21.0;
• onFirebaseDataNotificationIOS removed, as relied on upstream socket connection, which will be removed in Firebase 7 (https://firebase.google.com/support/release-notes/ios#fcm, Version 6.18.0 - February 25, 2020);
• Removed support for iOS 9;
• Small code clean up;
• @available used for runtime version check.