Fix Sonos group discovery

[amelchio]

Description:

I did not test discovery: very well with the recent Sonos rework and it turns out that two races are possible because it adds devices one by one:

• We could schedule_update_ha_state on a slave that was not yet async_added_to_hass.
• Slaves discovered after their group coordinator were not linked properly.

Example entry for configuration.yaml (if applicable):

discovery:

Checklist:

• The code change is tested and works locally.

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

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