fix(material/snack-bar): ensure that the snack bar always runs inside the NgZone

[crisbeto]

Adds an extra call to ensure that the snack bar is inside the NgZone. This is something that has come up several times in internal tests where some API call is stubbed out, pulling the snack bar outside the zone and causing tests to fail when we remove a change detection somewhere.

Note: I had a hard time reproducing this in our own tests, presumably because the fixture ends up pulling it back into the zone.

[andrewseguin]

This change doesn't work well with internal tests because they are so aggressive with flushing. This causes snackbar's setTimeout to finish before they can check expectations on the displayed snackbar.

Most tests can work around this by making sure they don't flush, but I wouldn't want to expect users to understand when and where they should feel safe flushing. See cl/437094895 for examples.

I'm worried this will make it difficult for users to easily write tests for the snackbar. Is there another way this can be implemented, or is there another fix we could try for these client tests that would be more intuitive?

[crisbeto]

The calls outside the NgZone seem to primarily break closing of the snack bar. I've reworked the PR to limit the impact only to the code paths related to closing.

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