Adjust screen brightness at idle; use new kernel function for getting

actual time since last tick; decrease run interval (make shorter) when
waiting for tasks to shutdown

PowerManagement.cpp

@@ -21,7 +21,7 @@ void sleep()
 void idle()
 {
   power_state = POWER_STATE_IDLE;
-  M5.Axp.ScreenBreath(7); 
+  M5.Axp.ScreenBreath(8); // 7-12
 }
 
 void wake()
@@ -41,7 +41,7 @@ int PowerManagement(int pid, unsigned int signal)
   // Accumulate time every time we run
   if ((signal & SIGNAL_START) == 0 && signal & SIGNAL_TICK)
   {
-    accumulator += Kernel_get_run_interval(pid);
+    accumulator += Kernel_get_last_tick_duration();
   }
 
   // But reset it whenever a button is pressed
@@ -71,6 +71,7 @@ int PowerManagement(int pid, unsigned int signal)
         }
         else
         {
+          Kernel_set_run_interval(pid, 50);
           Kernel_signal(SIGNAL_STOP);
         }
       }