Refactor and optimise the FCM functions

[bharat-1809]

The current code for FCM (Firebase Cloud Messaging) generates a huge amount of errors. Ideally, these are the exit cases that need to be addressed. Example: When the user is logged out, the token is also erased from the database but the cloud functions do not address this case and generate an error.

We need to make the code more robust and consider all exit cases.