altosdroid: Explicitly disconnect BT on termination or 'disconnect'

This adds an explicit message to the telemetry service telling it when
to stop trying to talk to the bluetooth device. Until this message is
received, the service will reconnect to the specified BT device.

That message is sent when you 'quit' the application, or when you 'disconnect'.

Signed-off-by: Keith Packard <keithp@keithp.com>