thermal-service: Decide on mechanism to determine if/when re-enter auto fan control

Fans currently support manual control (such as via requests from the host) or auto-temperature based control. Currently when the fan is instructed to enter manual control, it will just maintain the speed it was requested to run at. Some mechanism should be implemented to decide if auto-control should be resumed.

For example, the host may tell the fan to run at a low RPM, and if the host hangs or some other problem happens that prevents communication to the EC, the AP could overheat. Critical shutdown thresholds act as a last-resort fail-safe in this regard, but might be wise to still perhaps have the fan resume auto-control if temperature approaches some threshold before the critical threshold.