2.x: Single.subscribe() to report isDisposed() true on success/error

[akarnokd]

The ConsumerSingleObserver didn't report isDisposed consistently with its state.

Reported in #5160.

[codecov]

Codecov Report

Merging #5163 into 2.x will decrease coverage by -0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5163      +/-   ##
============================================
- Coverage     95.97%   95.95%   -0.02%     
+ Complexity     5655     5653       -2     
============================================
  Files           621      621              
  Lines         39960    39962       +2     
  Branches       5610     5610              
============================================
- Hits          38351    38345       -6     
- Misses          628      648      +20     
+ Partials        981      969      -12

Impacted Files	Coverage Δ	Complexity Δ
...vex/internal/observers/ConsumerSingleObserver.java	100% <100%> (ø)	7 <2> (ø)	❌
...in/java/io/reactivex/subjects/BehaviorSubject.java	83.93% <0%> (-6.74%)	55% <0%> (-1%)
...rnal/subscriptions/ArrayCompositeSubscription.java	93.33% <0%> (-6.67%)	17% <0%> (-1%)
.../operators/completable/CompletableConcatArray.java	93.75% <0%> (-6.25%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	86.44% <0%> (-6.08%)	55% <0%> (-1%)
...ernal/operators/maybe/MaybeTakeUntilPublisher.java	96% <0%> (-4%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableTakeLastTimed.java	96.29% <0%> (-2.78%)	2% <0%> (ø)
...main/java/io/reactivex/subjects/SingleSubject.java	95.23% <0%> (-2.39%)	38% <0%> (-1%)
...operators/observable/ObservableConcatMapEager.java	97.29% <0%> (-2.17%)	2% <0%> (ø)
.../internal/operators/maybe/MaybeTakeUntilMaybe.java	95.91% <0%> (-2.05%)	2% <0%> (ø)
... and 35 more

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