2.x: fix doOnSubscribe signalling Undeliv.Exception instead of just onError

[akarnokd]

This PR fixes the issue reported on StackOverflow where crashing the lambda in Flowable.doOnSubscribe calls the RxJavaPlugins.onError, which crashes the app and prevents calling the EmptySubscription.error on Android. (Desktop prints to the console and delivers the error normally.)

The bug was present in the Flowable, Observable and Completable. All 5 base types received the respective unit test to ensure correct behavior.

[codecov]

Codecov Report

Merging #5103 into 2.x will decrease coverage by -0.01%.
The diff coverage is 96.96%.

@@             Coverage Diff             @@
##                2.x   #5103      +/-   ##
===========================================
- Coverage     95.61%   95.6%   -0.01%     
- Complexity     5554    5562       +8     
===========================================
  Files           615     615              
  Lines         39685   39703      +18     
  Branches       5578    5583       +5     
===========================================
+ Hits          37944   37959      +15     
- Misses          757     762       +5     
+ Partials        984     982       -2

Impacted Files	Coverage Δ	Complexity Δ
...x/internal/observers/DisposableLambdaObserver.java	85.29% <100%> (+20.77%)	8 <4> (+2)	✅
...rnal/operators/flowable/FlowableDoOnLifecycle.java	100% <100%> (ø)	2 <ø> (ø)	❌
...nternal/operators/completable/CompletablePeek.java	96.87% <96.29%> (-3.13%)	2 <1> (ø)
...in/java/io/reactivex/subjects/BehaviorSubject.java	83.93% <ø> (-6.74%)	55% <ø> (-1%)
.../operators/completable/CompletableConcatArray.java	93.75% <ø> (-6.25%)	2% <ø> (ø)
...rnal/subscribers/SinglePostCompleteSubscriber.java	94.87% <ø> (-5.13%)	14% <ø> (-1%)
...rnal/subscriptions/DeferredScalarSubscription.java	93.84% <ø> (-4.62%)	27% <ø> (-1%)
...erators/completable/CompletableConcatIterable.java	95.91% <ø> (-4.09%)	2% <ø> (ø)
...reactivex/internal/operators/maybe/MaybeUsing.java	95.95% <ø> (-4.05%)	4% <ø> (ø)
...ernal/operators/maybe/MaybeTakeUntilPublisher.java	96% <ø> (-4%)	2% <ø> (ø)
... and 36 more

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