refactor: Splitting FCMService.kt file into multiple files; some cleanup too

[DanielDornhardt]

Hello everybody! 🤩,

This PR comes in preparation of some additional features we'd like to the Library from our codebase.

It is basically only moving things around to be able to work better with the code.

Description

We split FCMService.kt into some smaller service files and

Motivation and Context

We want to be able to merge some new fixes from our Repo back to the mainline.

Mostly this is related to VoIP Calls / allowing for Pushes to show a call dialog in the future on Android.

How Has This Been Tested?

• Automated tests completed
• Manually tested by our QA engineers

Types of changes

• Refactoring - Moving code around without any changes to the functionality

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[DanielDornhardt]

Also, all these changes have been made by the amazing @pppetrv !!! Me I'm just here right now creating the PR with the team.

Thank you @pppetrv !