Fix band select UI issue.

The issue was that scrolling past the top or bottom of the view caused the band
select rectangle to grow in the wrong direction. This was caused by adjusting
the origin of the band select rectangle based on the calculated number of pixels
to scroll instead of the actual number of pixels scrolled. There can be a
discrepancy when the calculated value is higher than the maximum that can be
scrolled (when the view is already scroll almost to the top or bottom of the
view).

Bug: 23081362

Change-Id: I343cdec89260ab0571a792009d0c08585ed41199