Should you trust your handset vendor/(and telco, if it's a phone that they've had a hand in)/Google? No, very probably not. The vendors do seem to care slightly more about bugs that might cause customer support calls or returns; and a lot less about security patches or providing vaguely recent versions of anything;
Well, if you're talking Apple - if the FBI has to call in for help to crack an iPhone, I'd say Apple cares about device security, at least. Sure they turned over iCloud data due to a subpoena, but anyone would have to do that.