Google Tracks Android app supports BT accessories like heart-rate and cadence sensors, but most available products in that category are actually ANT+, not BT. I hope it doesn't go that direction in general for this type of Android accessories because the BT devices (as I understand it) have to maintain a 2-way connection, where the ANT+ devices are connectionless. In other words, I think ANT+ accessories broadcast and don't really spend battery power caring whether any devices are listening, and they wouldn't have to be reset (re-paired? re-linked?) upon losing a connection.