2.x: Fix switchMap to indicate boundary fusion

[akarnokd]

Fix Flowable.switchMap and Observable.switchMap to request for boundary fusion in order to prevent a thread-confined but otherwise fusion-capable sub-sequence from running on the unintended thread.

Fixes: #5990

[codecov]

Codecov Report

Merging #5991 into 2.x will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5991      +/-   ##
============================================
+ Coverage     98.28%   98.29%   +<.01%     
  Complexity     6054     6054              
============================================
  Files           656      656              
  Lines         44078    44078              
  Branches       6118     6118              
============================================
+ Hits          43321    43325       +4     
+ Misses          229      223       -6     
- Partials        528      530       +2

Impacted Files	Coverage Δ	Complexity Δ
...rnal/operators/observable/ObservableSwitchMap.java	94.68% <100%> (ø)	3 <0> (ø)	⬇️
...internal/operators/flowable/FlowableSwitchMap.java	96.69% <100%> (+1.41%)	3 <0> (ø)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-5.98%)	2% <0%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	91.89% <0%> (-5.41%)	9% <0%> (-1%)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...ernal/operators/flowable/FlowableFromIterable.java	95.18% <0%> (-1.07%)	5% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	97.39% <0%> (-0.87%)	2% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	97.5% <0%> (-0.84%)	2% <0%> (ø)
.../operators/maybe/MaybeFlatMapIterableFlowable.java	97.54% <0%> (-0.82%)	2% <0%> (ø)
...x/internal/operators/flowable/FlowableFlatMap.java	88.94% <0%> (-0.79%)	4% <0%> (ø)
... and 21 more

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