fix: play through the alarm stream so the ring cuts through DND

DND silences the media stream by default, so playback on STREAM_MUSIC was
invisible during DND even though the notification showed. Switch to
STREAM_ALARM + USAGE_ALARM, which DND exempts by default, and restore/force
the alarm stream volume instead of media.

README.md

@@ -49,11 +49,14 @@ refuse to play any sound.** This is enforced at two points:
 Only after the user flips that toggle will a trigger actually ring.
 
 When access **is** granted, `RingService`:
-- saves the current music volume,
-- forces `AudioManager.STREAM_MUSIC` to max,
-- requests `AUDIOFOCUS_GAIN_TRANSIENT` using `USAGE_NOTIFICATION_RINGTONE` and
-  sets `AudioManager.mode = MODE_RINGTONE` so the sound cuts through DND,
-- plays for 30 s, then restores the volume and abandons focus.
+- saves the current **alarm-stream** volume,
+- forces `AudioManager.STREAM_ALARM` to max,
+- requests `AUDIOFOCUS_GAIN_TRANSIENT` using `AudioAttributes.USAGE_ALARM`,
+- plays on the **alarm stream** — the one stream Do Not Disturb does **not**
+  silence by default (media/music *is* silenced), which is what lets the ring
+  cut through DND,
+- plays for the configured number of seconds, then restores the volume and
+  abandons focus.
 
 ---