Sidenav: content area size not calculated properly with animation

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

#### What is the expected behavior?
The size of the content area is calculated correctly according to the width of the sidenav menu.

#### What is the current behavior?
The calculation is wrong. The margin-left value of the content area is inverted to the width defined in the animation.

#### What are the steps to reproduce?
https://stackblitz.com/edit/angular-etpys5
(to see the issue you have to click a few times on the expand button)

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
@angular/material@5.2.0
@angular/cdk@5.2.0
@angular/core@5.2.4
@angular/animations@5.2.4

Start by reproducing the issue in the linked StackBlitz example with the Angular Material 5.2.0 sidenav and clicking the expand button repeatedly. Trace the sidenav animation and content-area margin calculation, then verify that the content width and margin remain correct after repeated expand/collapse cycles.

Written by the indexing model from the issue text.