bug(mdc-dialog): afterClosed observable no longer in the NgZone

[mmalerba]

We got an internal bug report about this:

Observed behavior

Prior the MDC migration the following router navigation was happening inside of the NgZone.
After migration to MDC, this navigation is no longer happening in the NgZone.

Expected behavior

Callbacks for the afterClosed Observable should be in the NgZone

Steps to reproduce

1. Open a dialog
2. Subscribe to afterClosed()
3. Navigate within the callback, the Route complains that it is not in the NgZone

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