Police raids in China to stem billions of dollars in illicit foreign-exchange transactions have highlighted the continuing use of cryptocurrencies there despite Beijing’s prohibition of digital-asset trading.Alleged cases involving crypto flagged in May alone include an underground bank tied to 13.8 billion yuan ($1.9 billion) of illegal transfers, a gang implicated in about 2 billion yuan of unauthorized conversions and unlawful money changers that in some cases transacted more than 1 billion y...