feat(cdk/drag-drop): X and Y as percentage values instead of px

### Feature Description

We are developing a mobile app in which users should be able to move elements back and forth in a certain area. The position of the elements should obviously be identical for every user. However, this does not work since each user has a different smartphone and therefore a different screen width, as cdk/drag-drop works with px values. One should be able to switch from px to % in order to ensure the correct position regardless of the screen resolution (or the width of the boundary element).

Start by locating the cdk/drag-drop implementation that stores and applies X and Y positions, then inspect its existing pixel-based behavior and tests. Define how percentage values should be accepted and converted relative to the boundary element; done means positions remain consistent across different boundary widths and the behavior is covered by tests.

Written by the indexing model from the issue text.