Followup d4c133b: also make TargetEvents depend on ve.track()

On mobile, TargetEvents can be loaded without
ViewPageTarget.init, and in that case ve.track()
will be undefined.

Bug: 73038
Change-Id: I0b6bfb8b0fbdbfca57d48775b3634d70ca658f98
Merge "Followup d4c133b: also make TargetEvents depend on ve.track()"