According to this article on The Verge, Videostrong Technologies based in Shenzen, China, placed a backdoor into 1.3 million of their devices and used this to deploy a botnet:
www.theverge.com
The infected devices were used without owners' knowledge for "malicious activity," like ad fraud, data exfiltration, and routing unauthorized network traffic. Google claims the "backdoor and botnet were intentionally developed, deployed, and operated for financial gain"
One of Google’s Android partners turned 1.3 million set-top boxes into a botnet
A manufacturer intentionally hijacked its Android TV devices.