replace handler with timer for stopwatch tasks

[fatihergin]

fix https://github.com/SimpleMobileTools/Simple-Clock/issues/423

• replace "handler" with "timer" for stopwatch tasks as it relatively more resistance to background limitations: handler was running on main looper in here while timer runs on a new thread.

if the consistency still persist later, better solution would be moving all Stopwatch businesses to StopwatchService then notifying UI layer from there

• fix stopwatch -> update listener add/remove logic for better listener update consistency
• reduce notification update frequency as it doesn't make sense to update notification again and again with the same text: the time is displayed in seconds in notification bar, but the notification was being updated per 100ms.

[tibbi]

works fine in my quick test, thanks