Do not prevent default on pointermove

Instead, only prevent default on handled pointerdown events. This makes 
the `focus` condition work with interactions that involve dragging on 
touch devices.

test/spec/ol/interaction/draw.test.js

@@ -72,7 +72,8 @@ describe('ol.interaction.Draw', function() {
     const event = new PointerEvent(type, {
       clientX: position.left + x + width / 2,
       clientY: position.top + y + height / 2,
-      shiftKey: shiftKey
+      shiftKey: shiftKey,
+      preventDefault: function() {}
     }, {
       pointerType: 'mouse'
     });