Betterlife BL1500: Better ECO Mode handling

When you set eco mode the heater locks to 20ºC which isn't reflected in HA. 
This commit makes HA override the set temperature shown to 20ºC and after disabling Eco mode it goes back to displaying the previous temperature following the heater behavior.

custom_components/tuya_local/devices/betterlife_bl1500_heater.yaml

@@ -24,7 +24,10 @@ primary_entity:
         - constraint: preset_mode
           conditions:
             - dps_val: "2"
-              invalid: true
+              value: 20
+              range:
+                min: 20
+                max: 20
     - id: 4
       type: string
       name: preset_mode