feat(settings): let alarms and timers choose when they silence themselves

A ringing alarm and a finished timer used to stop after a fixed ten
minutes. Settings > Alarms and Settings > Timers now each have a "Silence
after" row with Google Clock's choices: 1, 5, 10, 15, 20 or 25 minutes,
or Never. Ten minutes stays the default, so nothing changes on upgrade.
The value is app-wide; a per-alarm override would need a new column, a
migration and an editor row, and is left out.

"Never" rings for an hour, then stops. The backstop is also what keeps a
phone left at home from ringing its battery flat, what bounds the ring
services' wake locks, and what stops a reboot hours later from resuming a
ring nobody was there for. A truly unbounded ring would give all of that
up, so Never maps to a one-hour hard stop, longer than any finite choice,
and the settings hint says so. Stored values clamp to 0..60 on read, on
write and from a backup.

The window is read everywhere the fixed constant was: the resolver's
ringing and late-snooze checks (which now take it without a default), the
backstop onFire arms, how late a snooze fire may arrive, whether an
interrupted ring is resumed after a reboot, upcoming(), the timer ring
policy, the lapsed-session check and the timer ring screen. Both ring
services' wake-lock timeouts now derive from the hard stop instead of a
hard-coded fifteen minutes.

When the resolver closes a ring (the window was shortened mid-ring, the
clock jumped, the alarm was switched off), the engine now also stops the
ring service and cancels the backstop. Before, it only cleared
ringingSince and left the sound to a backstop armed for the old window.

Both values are backed up as default_alarm_silence_after_minutes and
default_timer_silence_after_minutes; a file without them imports as ten
minutes.

Fixes #17

app/src/main/res/values/strings.xml

@@ -164,6 +164,16 @@
     <string name="challenge_math">Solve a sum</string>
     <string name="challenge_hold">Press and hold</string>
 
+    <!-- #17: how long a ring lasts before it stops on its own -->
+    <string name="settings_silence_after">Silence after</string>
+    <string name="silence_after_never">Never</string>
+    <plurals name="silence_after_minutes">
+        <item quantity="one">%1$d minute</item>
+        <item quantity="other">%1$d minutes</item>
+    </plurals>
+    <string name="settings_alarm_silence_after_hint">Applies to every alarm. An alarm nobody stops goes quiet after this long and counts as missed. Never still stops after an hour, so a phone left at home can\'t ring until its battery runs out.</string>
+    <string name="settings_timer_silence_after_hint">Applies to every timer. A finished timer nobody stops goes quiet after this long but stays finished, so you can still add a minute. Never still stops after an hour.</string>
+
     <!-- M5: the ringtone picker -->
     <string name="ringtone_picker_title">Alarm sound</string>
     <string name="ringtone_silent">Silent</string>