TokencodeBackend: Stop updating if onPause() is called from the update function

TokencodeWidgetService.onTokencodeUpdate() will call onPause() if there is
no more work left to do.  onPause() cancels all scheduled work on mHandler
but doUpdate() promptly schedules a new work item after
onTokencodeUpdate() returns, which is wrong.  Check for this condition.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>