Darwin: fix seek target precision in setPosition

The seek target was built by reusing the timescale of the item's
currentTime, which can be as coarse as 1. The requested position was
then truncated to whole seconds, e.g. seeking to 2571 ms landed at
2000 ms. Build the target with millisecond precision instead.

Unskip seekOnLoops and changeLoopsOnTheFly on Darwin.

Pick-to: 6.12
Change-Id: Ie7c0fba4e90b6e47f6fea11d8691e3640244378e
Reviewed-by: Tim Blechmann <tim.blechmann@qt.io>