isValidDeviceAddress
True iff address refers to a real selected device. Rejects null/blank and all legacy no-device sentinels ("n", "null", ".n", "default", case-insensitive) by delegating to normalizeAddress and isNoDeviceSentinel. Any input that buildDbName would collapse to DEFAULT_DB_NAME is rejected here too, so the foreground-service stay-alive decision and the DB name resolution can never diverge.