iOS: bugfix function portraitToPrimary()

The old implementation was wrong since it did not use the
screen's height (which was already in primary orientation) to
calculate what the new y value of the target rect (which was in
portrait) should be.

Change-Id: Ie5b2241119e244d099e06d85f69953c1d64979aa
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>