What is the realistic risk of a hardware backdoor in mainstream smartphones? I assume that there's nothing apps like these could do against those. Is that a r

What is the realistic risk of a hardware backdoor in mainstream smartphones? I assume that there's nothing apps like these could do against those. Is that a risk even worth worrying about at this point? If it is, is there anything that can be done about it? It is safe to trust, say, Google or HTC and their manufacturing chain? Further, if the handset is attacked and silently owned, all bets are off, too, right?