refactor(datepicker): adapt custom header demo for Ivy

[marclaval]

Same as #15458 for the ivy-2019 branch

[crisbeto]

LGTM

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     status "ci/circleci: api_golden_checks" is failing
     status "ci/circleci: bazel_build_test" is failing
     status "ci/circleci: build_release_packages" is failing
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     status "ci/circleci: tests_saucelabs" is failing
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[marclaval]

I've updated the example, PTAL

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