Merge "Add explanation to VHAL_HEARTBEAT property"

        | VehicleArea:GLOBAL),

    /**

     * Defines an event that VHAL signals to the car watchdog as a heartbeat.

     *

     * VHAL is supposed to write system uptime to this property at every 3 second.

     * Car watchdog subscribes to this property and checks if the property is updated at every 3

     * second. If it isn’t, car watchdog considers VHAL unhealthy and terminates it.

     * Defines an event that VHAL signals to car watchdog as a heartbeat.

     *

     * If VHAL supports this property, VHAL should write system uptime to this property at every 3

     * second. Car watchdog subscribes to this property and checks if the property is updated at

     * every 3 second. With the buffer time of 3 second, car watchdog waits for a heart beat to be

     * signaled up to 6 seconds from the last heart beat. If it isn’t, car watchdog considers

     * VHAL unhealthy and terminates it.

     * If this property is not supported by VHAL, car watchdog doesn't check VHAL health status.

     *

     * @change_mode VehiclePropertyChangeMode:ON_CHANGE

     * @access VehiclePropertyAccess:READ