2.X: Fix disposed LambdaObserver onError to route to global error handler

[hanschua]

Fix the behaviour of LambdaObserver when disposed to route to the global error handler

Fixes #6025

[codecov]

Codecov Report

Merging #6036 into 2.x will decrease coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##               2.x    #6036      +/-   ##
===========================================
- Coverage     98.3%   98.28%   -0.03%     
+ Complexity    6175     6172       -3     
===========================================
  Files          665      665              
  Lines        44729    44730       +1     
  Branches      6205     6205              
===========================================
- Hits         43973    43964       -9     
- Misses         222      229       +7     
- Partials       534      537       +3

Impacted Files	Coverage Δ	Complexity Δ
...o/reactivex/internal/observers/LambdaObserver.java	100% <100%> (ø)	14 <0> (ø)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-5.98%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelSortedJoin.java	92.7% <0%> (-2.92%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
...ava/io/reactivex/processors/BehaviorProcessor.java	96.86% <0%> (-2.25%)	60% <0%> (ø)
...x/internal/operators/flowable/FlowablePublish.java	97.02% <0%> (-2.13%)	11% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	98.61% <0%> (-1.39%)	57% <0%> (-1%)
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-1.2%)	2% <0%> (ø)
...java/io/reactivex/processors/UnicastProcessor.java	98.8% <0%> (-1.2%)	67% <0%> (-1%)
... and 21 more

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