When second source of data is available for a connectable, do not switch to it if CAN is not connected to

I noticed a few times where Sidecar will start showing the secondary voltage source instead of the one I had set before. I believe this happens if the scanner is turned on/connected to before the car is on.

Start by tracing Sidecar's connectable data-source selection when the scanner connects before the car, focusing on how CAN connection state affects the selected voltage source. Reproduce that startup order and verify that the previously selected source remains active until CAN is connected.

Written by the indexing model from the issue text.