Adjust flow type for accessType parameter in spyOn function

[ahnpnl]

Summary

Stricter type for flow type checking, it helps developers to know exactly what are the values expected by the function.
This PR makes clear the value of accessType input parameter of spyOn function is either get or set

Test plan
NA (not a bug)

[codecov-io]

Codecov Report

Merging #5354 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5354   +/-   ##
=======================================
  Coverage   61.29%   61.29%           
=======================================
  Files         205      205           
  Lines        6917     6917           
  Branches        4        4           
=======================================
  Hits         4240     4240           
  Misses       2676     2676           
  Partials        1        1

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[SimenB]

Thanks!

[github-actions]

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