applets/colorpicker: Only reset global current index from a real current item

The rule is simple: don't reset "currentIndex" if it does not belong to
you. There may be a race condition between different MouseAreas.

This came up in multiple places across Plasma software. Last thing I
remember, we fixed Pager (virtual desktops/activities) applet from
flickering the wrong tiles.

(cherry picked from commit f9748686)