Drag/drop placeholder doesn't work if placeholder is smaller than draggable list elements

#### Bug, feature request, or proposal:
Bug

#### What is the expected behavior?
The placeholder should be placed correctly, regardless of if the placeholder is the same size as the list elements or a different size.

#### What is the current behavior?
The placeholder is in the wrong spot in the list while dragging an element, but only if the placeholder is smaller (less height for a vertical list, or less width for a horizontal list).

#### What are the steps to reproduce?
Stackblitz: https://angular-ztyby3.stackblitz.io
Drag the top element down. Observe that the placeholder moves to the top. Then, move it back upwards, and observe that the placeholder moves to the bottom. This is the opposite of how the placeholder should work.

#### What is the use-case or motivation for changing an existing behavior?
It does not work as expected.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
@angular/*: 8.0.0
OS: Win10
TS: 3.4.5

#### Is there anything else we should know?

Comienza con la reproducción vinculada en StackBlitz y reproduce el comportamiento de arrastrar y soltar utilizando la configuración de Angular 8 indicada. Después, rastrea la implementación de arrastrar y soltar de Angular que posiciona el placeholder para elementos de lista de distintos tamaños. Se considera terminado cuando el placeholder permanece en la posición correcta al arrastrar un elemento hacia abajo y luego de vuelta hacia arriba, incluso cuando es más pequeño que los elementos de la lista.

Escrito por el modelo de indexación a partir del texto del issue.