[#75762464] Fix when touch/scroll events are suppressed on blockly blocks and background.

Yes - a touch and drag event on a block in the flyout should cause the block to be dragged, without scrolling the screen. Not preventing the default in this case causes scrolling to happen some of the time when dragging a block out of the flyout.

And yes - if a block is immovable, I would argue that it should not be a valid scroll target. Otherwise, a user who tried to drag an immovable block and instead scrolled the screen might be confused by the result.