Embed geofency platform into component

[rohankapoorcom]

Description:

This PR leverages #19948 to embed the Geofency device tracker platform under the component.

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• New dependencies have been added to the REQUIREMENTS variable (example).
• New dependencies are only imported inside functions that use them (example).
• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
• New files were added to .coveragerc.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[MartinHjelmare]

Just noticed that we don't use async_setup_entry in the geofency device tracker platform. Also noticed that there's no async_unload_entry in the device tracker base component.

It doesn't matter for the set up, but if trying to unload the entry, nothing will happen and the dispatch signal won't be removed. We should probably do something about this in the future.

[rohankapoorcom]

Also noticed that there's no async_unload_entry in the device tracker base component.

@MartinHjelmare does this mean the base component needs to switch to using EntityComponent?

[MartinHjelmare]

Not necessarily, although I would like that. Switching to EntityComponent would be a big refactor. For now, we could just add a class that collects the necessary setup and unload methods and has access to the DeviceTracker instance in the device tracker base component.

[rohankapoorcom]

Sounds good. I'll push something up once I get it working.

[rohankapoorcom]

Pushed up in #20237.