I'm confused as to why anyone would strongly believe something like this without any tangible evidence. For one thing, there are hundreds of hardware hackers that look for things like this, so it seems like if it were an issue one of them would have found evidence of it. For another, China knows this and wouldn't want to risk their global reputation in trade for something (spying on average citizens) that would have very little payoff compared to the long-term economic damage it would do.

Besides, as another commentator pointed out, if you're going to be worried about a country spying on you, it should at least be your own country.