deviceprovider: set the bus to non-flushing before calling klass->start

Not posting DEVICE_ADDED messages while a device provider is being started makes things awkward for applications, as they have to call get_devices() after starting the monitor.

This requires redundant code on the application side, and as far as I understand also could cause race conditions, when a device gets added between the calls to gst_device_monitor_start() and gst_device_monitor_get_devices(), causing the application to "see" the same device twice.

Marking as WIP because if we do decide to get this in, the device-monitor executable in -base will need updating.