Implement settings channel for the Linux shell

[robert-ancell]

#22435 Description

Implement settings channel for the Linux shell

Related Issues

flutter/flutter#65591

Tests

Tests added to confirm settings are sent on engine startup.

Manually tested by changing the following in gnome-control-center:

• Accessibility > Large Text (sets text scale to 1.25)
• Date & Time > Time Format (sets clock to 12/24hr).
• Appearance > Window colors (sets brightness to dark. Ubuntu extension, use gnome-tweak-tool for other systems).

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Reviewer Checklist

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Breaking Change

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[robert-ancell]

Due to a bug in the Flutter snap if you try the tests the setting values never change from their initial value. The workaround is to run the binary directly, and the fix will be in the snap.

[stuartmorgan-g]

Please add unit testing that the message is being sent on startup; see #22323 for an example.

[chinmaygarde]

@robert-ancell Any progress on the unit-tests requested by @stuartmorgan ?

[robert-ancell]

I'm currently working on a branch to add some base engine tests then will update this one with the new test for the settings channel.

[robert-ancell]

This includes the change in #22900, which should probably land before this one.

[stuartmorgan-g]

LGTM