Clearing connected message in stop fixes 7401152

We can stop before the service connected message is processed, in which case
we should clear the message.  Otherwise it can cause us to start up the UI
before the camera is open, causing a crash.

I was unable to reproduce this issue, but it theoretically will happen, and
the bug report shows all the symptoms of this series of events.

Change-Id: I60d295883b08ac58ccf0f3fc6d152b8b75fe9b18