Mtp: fix header formatting from previous commit, oops
Mtp: removed unused parameter in AbstractTrackList ctor

We take this opportunity to convert the ctor to protected, as there's
not much sense for it to be public given that the class is abstract.
Mtp: prevent earlier the use of NULL as MtpDevice in AbstractTrackList

In order to track down the source of bgo#733883, it's better to
throw ArgumentNullException earlier at creation time than later
causing a NullReferenceException when the field is going to be
used.

(In order to check against NULL only once, this commit chains 2
constructors together.)