Fixes column header drop indicator (#18925)

Fixes column header drop indicator (#18925)

Calculation of the frozen column width was wrong in multiselect mode.
It did not take into account the last frozen column, but added
the select column twice.
The same problem was in AutoScroller (as the same methods were
copy-pasted there), for which reason the autoscrolling was not
scrolling till the end.
This patch fixes the calculation error and removes copy-pasted code,
reusing the same code in both places.

Change-Id: I164e2fc96688088b620ad8785c533c593723f83e