fix: NavigateBack ObserveOn Router Scheduler

[RLittlesII]

What kind of change does this PR introduce?

Fixes a bug where NavigateBack was not firing NavigationStack changes.

What is the current behavior?

Resolves: #2286

What is the new behavior?

When NavigateBack is called the NavigationStack changes fire on the Router.Scheduler thread and the navigation works correctly.

What might this PR break?

Backwards navigation on Xamarin.Forms

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

[codecov]

Codecov Report

Merging #2315 into master will decrease coverage by 0.59%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #2315     +/-   ##
=========================================
- Coverage   55.32%   54.73%   -0.6%     
=========================================
  Files         113      113             
  Lines        4334     4334             
  Branches      660      660             
=========================================
- Hits         2398     2372     -26     
- Misses       1771     1797     +26     
  Partials      165      165

Impacted Files	Coverage Δ
...I.Winforms/WinformsCreatesObservableForProperty.cs	26.31% <0%> (-68.43%)	⬇️

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