Don't start/stop Bluetooth advertising if powered off

[cgst]

Attempting to

bluetoothManager.startAdvertising(nil)
bluetoothManager.stopAdvertising()

in triggerBluetoothStatusUpdate() outputs an error message on the iOS system console:

[CoreBluetooth] API MISUSE: <CBPeripheralManager: 0x15f1fb1a0> can only accept this command while in the powered on state

[nickoneill]

This is only called if the peripheral state is Unknown which is exclusive of knowing if the peripheral is on/off. I don't see a huge issue here other than the error message, so punting this until we refactor bluetooth to pending status in #113.