Keep minus button active while timer runs

[Xitee1]

Summary

• Minus button now stays visible after the timer starts so a running session can be shortened, not only extended.
• Pressing minus while running subtracts 5 minutes via a new ACTION_SUBTRACT_FIVE service action; the button is disabled when less than 5 minutes remain.
• SleepTimerService.subtractFiveMinutes() is a no-op if remaining ≤ 5 min and keeps totalDurationMillis consistent with remainingMillis.

Test plan

• Start a 15 min timer, tap minus once → countdown jumps to ~10 min and notification updates.
• Tap minus repeatedly until < 5 min remain → button becomes greyed out.
• Tap plus while running → still adds 5 minutes as before.
• Stop timer and adjust via minus/plus in idle state → unchanged behavior.

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