Use the backend to notify the QCamera state changes.

We were emitting the stateChanged() signal no matter what was actually
done by the backend. QCamera now emits this signal only if the backend
actually changed the state.

Task-number: QTBUG-35070
Change-Id: I9f32361cd815dbcc366ad30033692f9ae732ec46
Reviewed-by: Christian Stromme <christian.stromme@digia.com>