Fix Opus RX decoding, add foreground service, last-talker display

- Decode received Opus audio: handle svxlink self-delimited framing and
  provide monotonic PTS (c2.opus.decoder discards output when PTS == 0)
- Keep connection and playback alive in background via SvxService
  (foreground service, media playback type)
- Keep screen on while app is open
- Show last talker callsign persistently instead of clearing after 4s
- Bump versionCode to 2, disable lint on release builds

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

@@ -19,12 +19,13 @@
     <string name="status_connected">На связи</string>
     <string name="status_auth_failed">Ошибка авторизации</string>
     <string name="status_error">Ошибка: %1$s</string>
+    <string name="notif_connected">Соединение с рефлектором активно</string>
 
     <string name="btn_connect">Подключиться</string>
     <string name="btn_disconnect">Отключиться</string>
 
     <string name="talker_none">—</string>
-    <string name="talker_now">Сейчас говорит: %1$s</string>
+    <string name="talker_now">Последний: %1$s</string>
     <string name="talker_on_tg">группа %1$d</string>
 
     <string name="ptt_hint">Удерживайте кнопку, чтобы говорить</string>