[Preferences] Fix use of symbol for _PREFERENCE_THREADING

[vchuravy]

Fixes a mistake in #2476 where we can't store symbols in LocalPreferences.toml and thus have to do the conversion on either side.

[github-actions]

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.69%. Comparing base (a9cf615) to head (05aeb8e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/auxiliary/math.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2496       +/-   ##
===========================================
+ Coverage   79.36%   96.69%   +17.33%     
===========================================
  Files         509      509               
  Lines       42069    42107       +38     
===========================================
+ Hits        33387    40715     +7328     
+ Misses       8682     1392     -7290     

Flag	Coverage Δ
unittests	96.69% <0.00%> (+17.33%)	⬆️

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[ranocha]

Thanks