Disable grid scroll when dragging a widget

Grid scrolling and widget dragging gestures can compete with each other
somtimes causing a widget drag to get cancelled. This change disables
user scrolling while dragging a widget. Programatic scrolling is still
allowed for dragging a widget beyond visual boundaries.

Test: manually tested that dragging a widget is more stable, and a drag
      gesture does not get canceled once initiated. Also tested that
      dragging a widget to the edges of the screen still causes the grid
      to scroll.
Fix: 382355534
Flag: EXEMPT bug fix
Change-Id: Iae452a481ff20f96b57604dafad4600f101cdd4c