Add flashlight timer

[mgdx]

Summary

Adds a timer to the quick actions: pick a duration, the flashlight turns on and
switches itself off when the timer runs out — even if the app is closed in the
meantime.

The countdown is owned by a foreground service (TimerService) so it survives
the activity being destroyed. Responsibilities are split deliberately:

• MainActivity switches the light on, since it owns the light level and the
  matching UI (seekbar, level indicator);
• TimerService counts down and switches the light off again.

Every stop path — timer expired, "Stop" in the dialog, "Stop" in the notification,
"Off" button, morse/SOS mode — ends up in the same place, which then reports back
to the UI through a single callback.

Changes

• New TimerService, TimerDialog, dialog_timer.xml and ic_timer.xml.
• New "Timer" button in the quick actions, added to all three activity_main.xml
  variants (default, sw600dp, sw900dp).
• The notification is LOW priority, ongoing, opens the app when tapped and
  carries a "Stop" action. The remaining time is rendered by the system
  (setUsesChronometer + setChronometerCountDown), so the service does not wake
  up to redraw it.

Manifest / permissions

• Added FOREGROUND_SERVICE, FOREGROUND_SERVICE_SPECIAL_USE and
  POST_NOTIFICATIONS.
• TimerService is declared as specialUse — there is no dedicated foreground
  service type for the flashlight — together with the
  PROPERTY_SPECIAL_USE_FGS_SUBTYPE property that Android 14+ requires.