refactor(settings): remove the unused default timer duration

Settings > Timers offered a "Default duration", but nothing that creates
a timer ever read it: the keypad starts empty. The row, its preference,
ClockDefaults.timerDuration and the backup field are gone, and the
screen's hint and the hub's subtitle now only speak of the sound.

Backups no longer write default_timer_duration_millis. Older files that
carry it still import, since the codec ignores unknown keys.

Closes #5

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

@@ -294,13 +294,8 @@
     <string name="settings_alarms_subtitle">Sound, snooze and dismiss for new alarms</string>
     <string name="settings_alarm_defaults_hint">New alarms start with these. Each alarm can still change them in its editor.</string>
     <string name="settings_section_timers">Timers</string>
-    <string name="settings_timers_subtitle">Sound and length for new timers</string>
-    <string name="settings_timer_defaults_hint">New timers start with these.</string>
-    <string name="settings_timer_duration">Default duration</string>
-    <plurals name="settings_duration_minutes">
-        <item quantity="one">%1$d minute</item>
-        <item quantity="other">%1$d minutes</item>
-    </plurals>
+    <string name="settings_timers_sound_subtitle">Sound for new timers</string>
+    <string name="settings_timer_sound_hint">New timers ring with this sound. Each timer can still change it in its editor.</string>
     <string name="settings_section_clock">Clock</string>
     <string name="settings_clock_subtitle">Home time zone</string>
     <string name="settings_clock_hint">World clocks show how far each city is from your home time zone.</string>