App may hang if you press cancel while PCM is in recovery mode

[LegacyNsfw]

Seen with ELM-based devices, may or may not affect others.

The first loop in ElmDeviceImplementation.ReadElmLine doesn't check for \r characters. If the PCM is in recovery mode it will loop until the buffer is full, receiving the PCM's recovery-mode pings. This can take a very long time, and the app just appears hung.

Not sure what the side effects would be, though. Subsequent calls to ReadElmLine may or may not just pick up the read of the buffered messages. This would need a lot of testing.