[OOBE] Improve async loading of screens

Improve the asynchronous loading of OOBE screens by properly using
the Prioritized Task Scheduling API. The new calls replace the previous
ones that were using setTimeout and are not impacted by Tab Throttling
when the WebUI is still hidden. With this change, we can now enable asynchronous loading for the 'Add Person' flow as well.

Bug: b:294918131, b:287215163
Change-Id: I088ce33dc02c37b10296360df0cb6bd0842187a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4790364
Reviewed-by: Danila Kuzmin <dkuzmin@google.com>
Commit-Queue: Renato Silva <rrsilva@google.com>
Cr-Commit-Position: refs/heads/main@{#1184749}