Preferences: Turn stubthreshold back into a combo box

Currently it's an insanely tall list box. All the other settings on this
page use combo boxes, so we should be consistent.

After it was changed from 'selectorother' to 'select' in I88212703
and support for 'size' in HTMLSelectField was enabled in I7ac345e1,
the size defines the height of the select box instead of the width of
the text input field.

Bug: 68313
Change-Id: Id42a62e78fc2fee741bb02b88dc14cbb1b63635a
Merge "Preferences: Turn stubthreshold back into a combo box"