Add initial settings message to Windows embedding

[stuartmorgan-g]

Description

Sends the flutter/settings update message to the engine after starting
it. For now values other than 24-hour time preference are hard-coded,
but dark mode support can be added later.

Related Issues

Fixes flutter/flutter#65590

Tests

I added the following tests: A new test of engine initialization, including validating that this new message is sent.

Checklist

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• I read the contributor guide and followed the process outlined there for submitting PRs.
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• I updated/added relevant documentation.
• All existing and new tests are passing.
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Reviewer Checklist

• I have submitted any presubmit flakes in this PR using the engine presubmit flakes form before re-triggering the failure.

Breaking Change

Did any tests fail when you ran them? Please read handling breaking changes.

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change.

[stuartmorgan-g]

@cbracken Ping

[stuartmorgan-g]

At least for now I've turned off gtest_repeat (added in #12275) for the desktop embedding unit tests, because the lambda approach we're using is incompatible with multiple runs (I just hadn't noticed yet since we are just starting these tests). If we want to be able to support gtest_repeat, we'll need to completely revisit how we handle proctable replacement.

[cbracken]

lgtm

[chinmaygarde]

I've filed a presubmit flake report and rerun the presubmit.