fix: dismiss the overheat notification once the battery cools

[almothafar]

Closes #259

The problem

The high-temperature alert had hysteresis on firing but none on clearing:

• decideTemperature re-arms the persisted _temperature_alert_sent flag once the battery cools
  to threshold − TEMPERATURE_HYSTERESIS_C (3 °C).
• handleTemperature used that decision only to rewrite the boolean — it ignored the
  true → false transition.
• TEMPERATURE_NOTIFICATION_ID was never passed to NotificationManager.cancel() anywhere in the
  app, and the shared alertBuilder never sets setAutoCancel(true).

So the warning outlived the hot spell and sat in the shade until the user swiped it, reading as
"still overheating" on a phone that had long since cooled.

The change

Act on the true → false transition handleTemperature already computes and call the new
NotificationService.clearTemperatureAlert — the same "don't leave a stale alert alive" cleanup
FastDrainDetector already does for its own warning. No extra prefs read, no new state.

This covers both re-arm paths in decideTemperature:

• the battery cooled past the hysteresis point (the reported bug), and
• the user switched the high-temperature alert off in Settings while one was showing.

Staying inside the hysteresis band still holds the alert, so the notification cannot flap.

Deliberately out of scope: adding setAutoCancel(true) to the shared alertBuilder — that
would change tap-behaviour for every quiet-hours-aware alert, not just temperature.

Tests

Three cases added to BatteryLevelReceiverTest, which already mocks the NotificationService
statics:

• temperature_cooledBelowHysteresis_dismissesStaleAlert — hot tick then a cool tick, asserts
  clearTemperatureAlert called once.
• temperature_withinHysteresisBand_keepsAlertShown — negative case at 43 °C, asserts never().
• temperature_alertDisabledWhileShown_dismissesStaleAlert.

testDebugUnitTest, lintDebug and assembleDebug all pass on this branch.

Manual verification

Not run — no device available in this environment. Worth checking on hardware:

1. Lower the high-temperature threshold to 40 °C and heat the battery until the alert fires.
2. Let it cool; past 37 °C the notification should disappear on its own, no swipe.
3. Fire it again, then toggle "Notify on high temperature" off — it should clear immediately.

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