DisplayApp: Go to clock on sleep if no app loaded

When turning off the screen, if there is no actual app loaded (i.e. we
are still in the QuickSettings, Settings, or Launcher screens) we should
just reload the Clock app directly.

src/displayapp/DisplayApp.cpp

@@ -246,6 +246,10 @@ void DisplayApp::Refresh() {
         lcd.Sleep();
         PushMessageToSystemTask(Pinetime::System::Messages::OnDisplayTaskSleeping);
         state = States::Idle;
+        // Since the active screen is not really an app, go back to Clock.
+        if (currentApp == Apps::Launcher || currentApp == Apps::QuickSettings || currentApp == Apps::Settings) {
+          LoadScreen(Apps::Clock, DisplayApp::FullRefreshDirections::None);
+        }
         break;
       case Messages::GoToRunning:
         lcd.Wakeup();