Single-instance app, talker info in notification

- MainActivity singleTask, SplashActivity singleTop: re-launching from
  launcher or notification never creates a second instance
- Splash skips the 4s animation when the main screen is already open
- Foreground notification shows 'Говорит: CALLSIGN' / 'Последний:
  CALLSIGN'; tapping it brings the app back to the front

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

@@ -34,6 +34,8 @@
     <string name="status_auth_failed">Ошибка авторизации</string>
     <string name="status_error">Ошибка: %1$s</string>
     <string name="notif_connected">Соединение с рефлектором активно</string>
+    <string name="notif_talking">Говорит: %1$s</string>
+    <string name="notif_last_talker">Последний: %1$s</string>
 
     <string name="btn_connect">Подключиться</string>
     <string name="btn_disconnect">Отключиться</string>