Alarmdecoder component - add heartbeat attribute for RF sensors

The RF sensor state and attributes are updated when HA’s alarmdecoder receives a message from the alarmdecoder hub that a RF message from the sensor has been received. However, if there are no changes in state or attribute values, there is no record that the RF message was received.

For supervised RF sensors, the alarm panel will eventually report a problem if they don’t check in. However, for unsupervised sensors, there is no way to distinguish between ‘the sensor hasn’t changed’ and ‘the sensor hasn’t reported in’.

It would be useful to add an attribute to RF sensors, such as rf_heartbeat, that simply toggles between True/False every time an RF event is recorded from the sensor so there is a record of when the sensor last checked in.

I think it would be better to change HASS’s last updated time attribute for the sensors. This is a more general problem for all sensors (see If HASS would change the last updated time attribute whenever it receives a message even if the value isn’t changed, then every sensor would correctly show the last time it received a message.