Replace backup progress dialog with notification

app/src/main/java/eu/kanade/tachiyomi/data/notification/Notifications.kt

@@ -45,6 +45,12 @@ object Notifications {
     const val CHANNEL_UPDATES_TO_EXTS = "updates_ext_channel"
     const val ID_UPDATES_TO_EXTS = -401
 
+    /**
+     * Notification channel and ids used by the backup/restore system.
+     */
+    const val CHANNEL_BACKUP = "backup_channel"
+    const val ID_BACKUP = -501
+
     /**
      * Creates the notification channels introduced in Android Oreo.
      *