Improve opentherm_gw state detection

[mvn23]

Only show the platform in STATE_HEAT when the boiler is actually heating.

Description:

DATA_SLAVE_CH_ACTIVE does not necessarily indicate that the boiler is actually heating. We should check that DATA_SLAVE_FLAME_ON is true as well.

Related issue (if applicable): N/A

Pull request in home-assistant.io with documentation (if applicable): N/A

Example entry for configuration.yaml (if applicable):

N/A

Checklist:

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

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

• N/A

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

• No new files/dependencies

If the code does not interact with devices:

• N/A