Align hasCustomOnError behavior of CallbackCompletableObserver with L…

[hqzxzwb]

…ambdaObserver, ConsumerSingleObserver and so on

Background:
CallbackCompletableObserver.hasCustomOnError has a different implementation than other LambdaConsumerIntrospection implementations. This causes hasCustomOnError to return true even if one passes Functions.ON_ERROR_MISSING to Completable.subscribe(onComplete, onError). In contrast, if one passes Functions.ON_ERROR_MISSING to Single.subscribe(onComplete, onError), the ConsumerSingleObserver returns false from hasCustomOnError, and the case is the same with Single for Observable, Maybe and Flowable. This makes Completable a bit unnecessarily special.

Changes made:
Use the same strategy from other observable classes.

Might be breaking:
This change is making ABI changes on CallbackCompletableObserver. I am not sure if we need to maintain ABI stability even for classes in internal package. If it is needed, I am happy to add one more commit to keepCallbackCompletableObserver's ABI stable.

[codecov]

Codecov Report

Merging #7326 (30a0d89) into 3.x (804b8ef) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@ Coverage Diff @@## 3.x #7326 +/- ## ============================================ + Coverage 99.53% 99.55% +0.02% + Complexity 6784 6781 -3  ============================================ Files 751 751 Lines 47490 47482 -8 Branches 6378 6378 ============================================ + Hits 47269 47273 +4 + Misses 102 93 -9 + Partials 119 116 -3 

Impacted Files	Coverage Δ
...in/java/io/reactivex/rxjava3/core/Completable.java	100.00% <100.00%> (ø)
...nternal/observers/CallbackCompletableObserver.java	100.00% <100.00%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0.00%> (-4.66%)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	94.44% <0.00%> (-3.97%)	⬇️
...nternal/operators/parallel/ParallelReduceFull.java	93.06% <0.00%> (-1.99%)	⬇️
...operators/flowable/FlowableFlatMapCompletable.java	98.71% <0.00%> (-1.29%)	⬇️
...3/internal/operators/flowable/FlowablePublish.java	99.00% <0.00%> (-1.00%)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	91.54% <0.00%> (-0.71%)	⬇️
.../reactivex/rxjava3/observers/BaseTestConsumer.java	99.46% <0.00%> (-0.54%)	⬇️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	96.89% <0.00%> (-0.52%)	⬇️
... and 47 more

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