feat(timers): give timers their own vibration and gradual volume

Timers vibrated by the alarm vibration default and never ramped. Both are
now the timers' own app-wide settings, next to the default timer sound in
Settings > Timers: a Vibrate switch and the same Gradual volume picker
alarms use. Timer.resolveSettings reads them instead of the alarm default
and the old TimerRing.RAMP_SECONDS constant.

A fresh install starts with timer vibration off, as Google Clock does, and
no ramp. An existing install keeps what its timers did: a one-shot DataStore
migration writes the new key from the alarm vibration default before the
first read, so nobody's kitchen timer silently stops buzzing on update. A
fresh install is told apart by an empty store (anything that has run once
has written its boot id); the migration writes the default there too, so it
never runs twice.

Backups carry both new fields. A file without them restores the alarm
vibration default as the timers' (what that install's timers did) and a
flat ring.

Silent plus vibration off still signals: Silent gives the player no
sources, and its onSilenced callback starts the vibrator whatever the
setting says. The settings row says so.

Fixes #22

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

@@ -298,8 +298,9 @@
     <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_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_timers_subtitle">Sound, vibration and gradual volume</string>
+    <string name="settings_timer_defaults_hint">How timers ring. A timer can still pick its own sound in its editor.</string>
+    <string name="settings_timer_vibrate_hint">A timer set to silent vibrates either way</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>