2.x: Make Flowable.fromCallable consistent with the other fromCallables

[akarnokd]

Flowable.fromCallable is one of the oldest 2.x operators and was overlooked when the handling of beyond-cancellation error delivery, aka the undeliverable exception handling was implemented across RxJava. This PR resolves this shortcoming by making it consistent with its sibling implementations.

Resolves: #6156

[codecov]

Codecov Report

Merging #6158 into 2.x will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6158      +/-   ##
============================================
+ Coverage     98.22%   98.24%   +0.02%     
- Complexity     6196     6200       +4     
============================================
  Files           667      667              
  Lines         44857    44859       +2     
  Branches       6213     6214       +1     
============================================
+ Hits          44061    44073      +12     
+ Misses          251      248       -3     
+ Partials        545      538       -7

Impacted Files	Coverage Δ	Complexity Δ
...ernal/operators/flowable/FlowableFromCallable.java	100% <100%> (ø)	4 <0> (+1)	⬆️
...rnal/operators/flowable/FlowableFlatMapSingle.java	93.47% <0%> (-2.18%)	2% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...ex/internal/operators/flowable/FlowableCreate.java	96.12% <0%> (-1.62%)	6% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	99.11% <0%> (-0.89%)	10% <0%> (-1%)
...l/operators/observable/ObservableTimeoutTimed.java	99.15% <0%> (-0.85%)	3% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-0.75%)	2% <0%> (ø)
...vex/internal/operators/flowable/FlowableCache.java	97.94% <0%> (-0.69%)	10% <0%> (-1%)
...rnal/operators/observable/ObservableSwitchMap.java	94.68% <0%> (-0.54%)	3% <0%> (ø)
...main/java/io/reactivex/subjects/ReplaySubject.java	97.69% <0%> (-0.42%)	50% <0%> (ø)
... and 19 more

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