fix: label the ringtone preview button as a preview

TalkBack read the play button as the tone's name alone, right after the
row that already said it.

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

@@ -169,6 +169,7 @@
     <string name="ringtone_silent">Silent</string>
     <string name="ringtone_silent_summary">Vibration only</string>
     <string name="ringtone_device_default">Device default alarm sound</string>
+    <string name="ringtone_preview_play">Preview %1$s</string>
     <string name="ringtone_preview_stop">Stop %1$s</string>
     <string name="ringtone_pick_file">Choose from files\u2026</string>
     <string name="ringtone_none_available">This device has no alarm sounds. Clockula will use the system default.</string>