2.x: Upgrade the algorithm of Observable.timeout(time|selector) operators

[akarnokd]

This PR improves the algorithms of the Observable.timeout(time) and Observable.timeout(selector) operators to match their Flowable counterparts implemented in #5661.

In addition, coverage of the Flowable counterparts have been improved by synchronizing the unit tests.

Note that they were the last users of ObserverFullArbiter and FullArbiterObserver which are now removed.

Closes #5882.

[codecov]

Codecov Report

Merging #5886 into 2.x will increase coverage by 0.04%.
The diff coverage is 99.48%.

@@             Coverage Diff              @@
##                2.x    #5886      +/-   ##
============================================
+ Coverage     97.41%   97.45%   +0.04%     
+ Complexity     5976     5946      -30     
============================================
  Files           657      655       -2     
  Lines         43960    43877      -83     
  Branches       6128     6109      -19     
============================================
- Hits          42824    42761      -63     
+ Misses          346      343       -3     
+ Partials        790      773      -17

Impacted Files	Coverage Δ	Complexity Δ
...ternal/operators/observable/ObservableTimeout.java	100% <100%> (+3.47%)	3 <0> (ø)	⬇️
...l/operators/observable/ObservableTimeoutTimed.java	99.15% <98.7%> (+5.55%)	3 <0> (-1)	⬇️
.../io/reactivex/internal/functions/ObjectHelper.java	94.73% <0%> (-5.27%)	20% <0%> (-1%)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...ernal/operators/observable/ObservableInterval.java	96.42% <0%> (-3.58%)	3% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	91.3% <0%> (-2.9%)	2% <0%> (ø)
...in/java/io/reactivex/subjects/BehaviorSubject.java	95.76% <0%> (-2.65%)	54% <0%> (ø)
...tivex/internal/schedulers/TrampolineScheduler.java	92.3% <0%> (-2.57%)	6% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...perators/single/SingleFlatMapIterableFlowable.java	95.83% <0%> (-2.5%)	2% <0%> (ø)
... and 26 more

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[vanniktech]

Really nice.