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

首先，在链接的 StackBlitz 示例中使用 Angular Material 5.2.0 sidenav，并反复点击 expand 按钮，以重现该问题。跟踪 sidenav 动画和内容区域边距的计算，然后验证在反复执行 expand/collapse 周期后，内容宽度和边距仍然正确。

由索引模型根据 Issue 内容生成。