[Sidenav] Issue when container and sidenav in different components

#### Bug, feature request, or proposal:
I have the template like this:
``
` `
`...`
``

`` is:
``
` ...`
``

When I have my sidenav in two components like this, the mode "**side**" is not working anymore. The mode switch to "**over**".
When I have all in the same component, it's working.

I think it's due to a another tag between and and it break the css rules.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular Material 5.0.1

Start by reproducing the reported template with Angular Material 5.0.1, placing the mat-sidenav-container and mat-sidenav in separate components, then compare it with the same-component arrangement. Done means mode="side" remains effective when the sidenav is nested in my-component instead of switching to mode="over".

Written by the indexing model from the issue text.