fix(drag-drop): avoid interfering with element clicks (#13152)

Resolves a long-standing TODO about starting the drag&drop sequence after the user has dragged over a certain amount of pixels, rather than starting it immediately on `mousedown`/`touchstart`. This prevents random clicks and taps on the element from being interpreted as dragging and avoids potentially interrupting the native click events on children of `CdkDrag`.

Author: crisbeto