We have already heard of TVs doing this . Pen testers were able to build an app that turns a smartphone into an eavesdropper , mostly from re-used bits of code . More details on the BBC website today.
Personally I'd solder-remove any unwanted Microphone or network card on the TV. But with a smartphone that is quite simpler: read the terms before installing an "app". However, it's quite notorious that Google will allow some malicious program to be displayed on the store, so I'd say don't trust anything that is new and untested, keep only the basic apps.