feat(expansion-panel): align with 2018 material design spec

[crisbeto]

Aligns the expansion panel with the latest Material Design spec.

[josephperrott]

LGTM

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     status "ci/circleci: build" is failing
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[jelbourn]

LGTM

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