fix: improved stopwatch service and notification

[SuhasDissa]

Improvements

• Use a bound service for stopwatch and move it to foreground only when a stopwatch is running ¹
• Move the service connection logic into the MainActivity ²
• Use a simple integer value to store the stopwatch progress
• Click the notification to open stopwatch screen

Demo

Old Behavior	New Behavior
Screen_recording_old.mp4	Screen_recording_new.mp4

Other tests

I started the stopwatch. Cleared the app from task view and waited for about a half an hour with the device screen turned off. The stopwatch service managed to survive that whole time without crashing.

Footnotes

1. This should be more responsive than starting and stopping the service for every stopwatch run ↩

2. This prevents leaking a context object in the viewmodel ↩

[Bnyro]

Please move that to a const or variable somewhere, so it's easier to overview which ones we already use and which not. Same for stopAction().

[Bnyro]

Works as expected from my testing 👍