[MWF] Fix clipping of last line of dropdown (#2462)

The height we calculate is the inner height, so we need to add the top
and bottom borders when setting the height of the listbox. The width
however is the outer width so we need to subtract the left and right
borders when setting the client width.
Merge pull request #951 from ermshiperete/bug-xamarin-2462

[MWF] Fix clipping of last line of dropdown (#2462)