Client: Cleanup mouse state after drag

Fixes an issue where dragging with the mouse would cause the next touch event
to not generate a synthesized mouse press event.

The touchDrag test can now be run directly after the mouseDrag test without
failing.

Task-number: QTBUG-56187
Change-Id: I53cc5f90fc8d8672936b23f54a017687d41c31fc
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>