fix(dialog,bottom-sheet): invert backdrop color on dark themes

[crisbeto]

Similarly to how we handle the sidenav backdrop and ripples, these changes invert the color for the dialog and bottom sheet backdrops in dark themes.

Note: this is a resubmit of #13065.

[ngbot]

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

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[jelbourn]

@crisbeto it looks like this is changing the backdrop color for light themes as well, which is causing a lot of screenshot diffs

[crisbeto]

AFAIK this isn't in the spec. Closing.

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