modify the bound of dragging

- shorten the dragging distance
- lower the minimum alpha
change text alpha along with outside circle
fix the dragging out of the bound problem

- the problem is caused by the touch event dispatching mechanisam:
  once the user move the dragging finger out of current view, the
  previous checking condition `inMetricPort()` won't meet. Hence
  the ACTION_UP event is not properply dispatched to the child.
- by adding a variable to indicate the dragging process is still on,
  the parent can correctly dispatch the touch event now.
allow only one BriefClock to be set at one time