fix(expansion): show header focus state when expanded

[jelbourn]

The expansion-panel currently shows the header focus state when
collapsed. This is an a11y problem because there's no indicator of focus
state when expanded.

[crisbeto]

LGTM

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     status "ci/circleci: tests_local_browsers" is failing
     status "ci/circleci: tests_browserstack" is pending
     status "ci/circleci: tests_saucelabs" is pending

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[Splaktar]

It looks like this fixes #14569?

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