2.x: Don't dispose the winner of {Single|Maybe|Completable}.amb()

[akarnokd]

This PR fixes the Single.amb, Maybe.amb and Completable.amb operators to not dispose the winner source, causing potential interruptions as a side-effect on the current thread.

The fix is to delete the winner from the composite before disposing the rest to avoid the interruption race.

Unit tests were added to verify this behavior on all amb implementations. Flowable and Observable were already working correctly.

Fixes: #6373

[vanniktech]

TIL: about delete()

[codecov]

Codecov Report

Merging #6375 into 2.x will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6375      +/-   ##
============================================
+ Coverage     98.24%   98.27%   +0.02%     
- Complexity     6282     6288       +6     
============================================
  Files           673      673              
  Lines         45023    45035      +12     
  Branches       6226     6226              
============================================
+ Hits          44235    44259      +24     
+ Misses          248      243       -5     
+ Partials        540      533       -7

Impacted Files	Coverage Δ	Complexity Δ
...internal/operators/completable/CompletableAmb.java	100% <100%> (ø)	11 <0> (ø)	⬇️
...o/reactivex/internal/operators/maybe/MaybeAmb.java	100% <100%> (ø)	11 <0> (+1)	⬆️
...reactivex/internal/operators/single/SingleAmb.java	100% <100%> (ø)	10 <0> (ø)	⬇️
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-3.81%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	90.19% <0%> (-3.27%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	96.86% <0%> (-2.25%)	60% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	95.83% <0%> (-1.67%)	2% <0%> (ø)
...java/io/reactivex/processors/UnicastProcessor.java	98.8% <0%> (-1.2%)	67% <0%> (-1%)
...perators/observable/ObservableMergeWithSingle.java	99.06% <0%> (-0.94%)	2% <0%> (ø)
... and 27 more

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