fix: WhenActivated not fired when in ViewModelViewHost

[glennawatson]

Fixes #2021

[codecov]

Codecov Report

Merging #2023 into master will decrease coverage by 0.6%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2023      +/-   ##
==========================================
- Coverage   60.78%   60.18%   -0.61%     
==========================================
  Files         115      116       +1     
  Lines        4580     4448     -132     
  Branches      656      638      -18     
==========================================
- Hits         2784     2677     -107     
+ Misses       1614     1598      -16     
+ Partials      182      173       -9

Impacted Files	Coverage Δ
src/ReactiveUI/EventManagers/WeakEventManager.cs	60.43% <0%> (-3.33%)	⬇️
src/ReactiveUI/ReactiveCommand/ReactiveCommand.cs	79.32% <0%> (-0.89%)	⬇️
...rvableCollectionChangedToListChangedTransformer.cs	86.66% <0%> (ø)
src/ReactiveUI/Legacy/ReactiveDerivedCollection.cs	40.67% <0%> (+0.4%)	⬆️
...eactiveUI/ReactiveCommand/ReactiveCommandMixins.cs	100% <0%> (+3.77%)	⬆️
...tiveUI/ReactiveObject/IReactiveObjectExtensions.cs	100% <0%> (+4.54%)	⬆️
src/ReactiveUI/Comparers/OrderedComparer.cs	83.33% <0%> (+19.69%)	⬆️

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