Fix HomeMatic availability detection

[danielperna84]

Description:

Currently the transition from unavailable to available for devices that are offline during startup doesn't work reliably. pyhomematic handles the availability state internally though, and this change makes use of that. This should have been fixed with #16202, but for some reason it still didn't work.

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

[pvizeli]

That was my origin code. The problem is, that the library don't update this flag internal. I never found out why. Do you test that local?

This current code work also only if the device send data otherwise it stay unavailability.

If that now work, it would be the best solution instead my workaround

[danielperna84]

The problem is, that the library don't update this flag internal

It actually does. At least in the following scenario the change in this PR works as intended:

1. Stop HASS
2. Remove Battery from Wall Thermostat
3. Control Thermostat from CCU (the generates the Kommunikationsstörung message -> unreachable)
4. Start HASS
5. Entity gets created, but as unavailable
6. Plug in Battery into device
7. Event: UNREACH: False gets distributed
8. pyhomematics internal callback sets hmdevice._unreach to False
9. Any event for that device leads to comparing the HASS-availability to the pyhomematic-state, which turns the HASS entity to available.

Before my change this did not work. I can't easily test the opposite direction. Removing the battery doesn't send the UNREACH event (at least not within the 2 minutes I have waited). But in another issue at pyhomematic a user reports his IP device turning unavailable when switching it (because of exhausted duty cycle). So I assume he will actually see the UNREACH: True event. So the test would be, if for him this also sets the HASS entity to not available.

In any case this is an improvement (for me) because devices that were unavailble at startup eventually go online later on. Until now they would stay offline until I restart HASS.