wasp: clock: Reduce battery meter redraws

The ADC on nRF doesn't run precisely stable which means the battery
meter can flicker if updated too often. This will eventually
be fixed by the framework but, for now, let's just force the
update rate to be fairly slow.

wasp/clock.py

@@ -25,14 +25,16 @@ class ClockApp(object):
 
         display.fill(0)
         display.rleblit(fonts.clock_colon, pos=(2*48, 80), fg=0xb5b6)
+        self.on_screen = ( -1, -1 )
         self.update(watch)
         self.meter.draw()
 
-
     def update(self, watch):
         now = watch.rtc.get_time()
         if now[0] == self.on_screen[0] and now[1] == self.on_screen[1]:
-            self.meter.update()
+            if now[1] % 2 == 0:
+                self.meter.update()
+            return False
 
         display = watch.display
         display.rleblit(DIGITS[now[1]  % 10], pos=(4*48, 80))
@@ -42,3 +44,4 @@ class ClockApp(object):
         self.on_screen = now
 
         self.meter.update()
+        return True