feat(paginator): default page size to first option

[andrewseguin]

If no page size is provided, use the first option in the list of options.

Note that markForCheck has been added to each @Input since they may be changed directly rather than through templating. This fixed the test should be able to go to the previous page which changed pageIndex to 1 but the paginator never re-evaluated its template because the input was changed directly in the MdPaginator.

Fixes #5767

[jelbourn]

LGTM

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