2.x: add Single.unsubscribeOn()

[akarnokd]

This PR adds the Single.unsubscribeOn() operator.

Related: #5300.

[JakeWharton]

Looks like this landed on the wrong method

[akarnokd]

Thanks, fixed.

[codecov]

Codecov Report

Merging #5302 into 2.x will decrease coverage by 0.07%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5302      +/-   ##
============================================
- Coverage     96.17%   96.09%   -0.08%     
  Complexity     5756     5756              
============================================
  Files           628      629       +1     
  Lines         41085    41112      +27     
  Branches       5703     5705       +2     
============================================
- Hits          39514    39508       -6     
- Misses          613      629      +16     
- Partials        958      975      +17

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	99.33% <100%> (ø)	132 <1> (+1)	⬆️
...internal/operators/single/SingleUnsubscribeOn.java	100% <100%> (ø)	2 <2> (?)
...rnal/subscriptions/ArrayCompositeSubscription.java	93.33% <0%> (-6.67%)	17% <0%> (-1%)
...internal/operators/completable/CompletableAmb.java	94.91% <0%> (-5.09%)	10% <0%> (-1%)
...erators/completable/CompletableConcatIterable.java	95.91% <0%> (-4.09%)	2% <0%> (ø)
...ernal/operators/maybe/MaybeTakeUntilPublisher.java	96% <0%> (-4%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	93.28% <0%> (-2.99%)	2% <0%> (ø)
...activex/internal/operators/single/SingleCache.java	97.05% <0%> (-2.95%)	23% <0%> (-1%)
...ex/internal/operators/maybe/MaybeTimeoutMaybe.java	95.58% <0%> (-2.95%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelSortedJoin.java	92.75% <0%> (-2.9%)	2% <0%> (ø)
... and 35 more

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