Windows: Add dark theme support.

[moko256]

This PR adds the dark theme support for both windows and winuwp engine, based on flutter/flutter#54612.

On Win32: using a registry value that is not documented. (I tested on Windows 10.0.19043.1165)
On UWP: using default background color. (I couldn't find WinRT API that returns dark/light directly, without xaml package.)

TODO:

• Provide the theme value
• Notify when the theme value is changed.

Flutter PR for win32: flutter/flutter#88520.
This PR will close flutter/flutter#54612.
This PR will complete one item in flutter/flutter#70214.

No changes in flutter/tests.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the [CLA].
• All existing and new tests are passing.

[chinmaygarde]

cc @cbracken

[cbracken]

Just one API nit, other than that, this looks good to me!

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[cbracken]

[cbracken]

Applied a quick one-line format fix. Looks great -- thanks! A lot of people will be excited to see this land! Apologies for the delay in getting to this; was out of the office for a bit.

[moko256]

Thanks for your reviewing. (and I'm sorry I forgot to format code.) I'm excited too!

[google-cla]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

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[cbracken]

Rebased and pushed to pick up the same API key change as last time.