bluetoothd always send the GetManagedObjects() reply messages with the
objects array argument following an in-depth order starting from the
root. That means parents will always be known at the time their children
objects are parsed, if clients parse the objects in the same order they
appear in the array, as we do in PulseAudio.

This commit tries to protect PulseAudio in the case bluetoothd changes
that behavior for some reason. It hasn't been tested, since this
situation never occurs.