For #12201 - forced sync layout callbacks to be on the main thread

[BranescuMihai]

The account manager from AC works similar to the store that we have (so it has states and a reducer), however it delivers the result to consumers via old-school observer pattern, and since the reducer is on a different thread, all consumers are notified on that thread as well.

This consumer needs to perform UI changes, so it's required to be on main thread for that to happen. I chose to set only this consumer on the main thread and not switch context in the account manager reducer, because some consumers do computation and should remain on the separate thread.

Pull Request checklist

• Tests: This PR includes thorough tests or an explanation of why it does not
• Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features. In addition, it includes a screenshot of a successful accessibility scan to ensure no new defects are added to the product.

After merge

• Milestone: Make sure issues finished by this pull request are added to the milestone of the version currently in development.

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[codecov-commenter]

Codecov Report

Merging #12543 into master will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master   #12543      +/-   ##
============================================
- Coverage     23.66%   23.62%   -0.04%     
  Complexity      798      798              
============================================
  Files           377      377              
  Lines         15478    15483       +5     
  Branches       2020     2020              
============================================
- Hits           3663     3658       -5     
- Misses        11507    11515       +8     
- Partials        308      310       +2     

Impacted Files	Coverage Δ	Complexity Δ
...in/java/org/mozilla/fenix/sync/SyncedTabsLayout.kt	7.50% <0.00%> (-1.08%)	0.00 <0.00> (ø)
...mponents/searchengine/FenixSearchEngineProvider.kt	58.55% <0.00%> (-4.51%)	14.00% <0.00%> (ø%)

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[eliserichards]

Looks great!