fix(material/core): expose mixin mentioned in the elevation guide

[crisbeto]

We were mentioning the mat-elevation-transition mixin, but the new API wasn't exposing it. Also updates the imports to the new API.

[crisbeto]

Caretaker note: based on the discussion above, I've renamed the mat-elevation-transition-property-value function to mat-private-elevation-transition-property-value. The 4 usages inside g3 should either be switched to the hardcoded value from inside the function, or temporarily moved over to use the private function until they can be migrated.

[jelbourn]

lgtm

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