2.x: concat to report isDisposed consistently with termination

[akarnokd]

This PR makes sure the Disposable of a concat operator reports true when the sequence terminates.

Reported in #5439

[codecov]

Codecov Report

Merging #5440 into 2.x will decrease coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5440      +/-   ##
============================================
- Coverage     95.96%   95.93%   -0.04%     
- Complexity     5793     5794       +1     
============================================
  Files           631      631              
  Lines         41297    41302       +5     
  Branches       5744     5744              
============================================
- Hits          39632    39622      -10     
- Misses          663      667       +4     
- Partials       1002     1013      +11

Impacted Files	Coverage Δ	Complexity Δ
...rnal/operators/observable/ObservableConcatMap.java	97.52% <100%> (+0.05%)	5 <0> (ø)	⬇️
...nternal/operators/observable/ObservableCreate.java	78.63% <0%> (-18.81%)	2% <0%> (ø)
.../operators/completable/CompletableConcatArray.java	93.75% <0%> (-6.25%)	2% <0%> (ø)
.../internal/operators/observable/ObservableHide.java	90% <0%> (-5%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableSkipLastTimed.java	93.87% <0%> (-4.09%)	2% <0%> (ø)
...io/reactivex/internal/util/BackpressureHelper.java	95.91% <0%> (-4.09%)	21% <0%> (-1%)
.../internal/disposables/ListCompositeDisposable.java	96% <0%> (-4%)	33% <0%> (-2%)
...l/operators/observable/ObservableFlatMapMaybe.java	84.96% <0%> (-3.93%)	2% <0%> (ø)
...vex/internal/operators/flowable/FlowableTimer.java	92.85% <0%> (-3.58%)	2% <0%> (ø)
...internal/operators/completable/CompletableAmb.java	94.91% <0%> (-3.39%)	10% <0%> (-1%)
... and 39 more

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