Implement merged UI and platform thread

[seungsoo47]

Original issue: flutter/flutter#150525

This PR lets the embedder run both with and without UI and platform thread merged.
Thread merging is controlled through metadata in tizen-manifest.xml as follows and the default value is currently false.

<ui-application ... >
    <metadata key="http://tizen.org/metadata/flutter_tizen/enable_merged_platform_ui_thread" value="true" />
</ui-application>

[JSUYA]

Original issue: flutter/flutter#150525

In 3.32, this feature by default on iOS, Android, in 3.33 beta, it by default on Windows and macOS, and In Linux, it is WIP. This setting will likely be enabled by default soon.
I think we can support a flag to disable this when it becomes the default on Linux in the stable version.
(We have to be very careful about providing metadata keys.)

[seungsoo47]

As discussed offline, we can merge this patch after this feature is enabled on Linux.