DeskClock: Fix stopwatch notification

* Currently, pressing the stopwatch notification does nothing
  (but add some logs...)
* Since Android 12, notification trampolines are restricted:
  https://developer.android.com/about/versions/12/behavior-changes-12#notification-trampolines
* We need to start an activity directly, therefore move the relevant
  code to DeskClock.java directly

* Fixes: https://gitlab.com/LineageOS/issues/android/-/issues/5586

Test:
- Start the stopwatch
- Minimize app
- Wait for notification and tap it -> DeskClock reopens at stopwatch tab
- Close app
- Wait for notification and tap it -> DeskClock reopens at stopwatch tab

Change-Id: I4dc23e1f134b9c69f5df361d4403b946e84131c4