DeskClock: Fix timer notification

* Currently, pressing the timer 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/5558

Test:
- Start a timer (with enough time for the rest of the test)
- Minimize app
- Wait for notification and tap it -> DeskClock reopens at timer tab
- Close app
- Wait for notification and tap it -> DeskClock reopens at timer tab

Change-Id: I18c742b0b6de840776931a4fb42c2420c4c9fae1