2.x: Add Single.delay overload that delays errors

[PhilGlass]

Much like #5382, I discovered when trying to fake a slow/flaky backend that Single.delay doesn't delay errors.

This PR adds an overload that does, bringing Single.delay in line with Observable.delay. It also corrects the javadoc for existing overloads (#5409 only updated 1/2).

[codecov]

Codecov Report

Merging #5616 into 2.x will decrease coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5616      +/-   ##
============================================
- Coverage     96.27%   96.22%   -0.06%     
+ Complexity     5844     5841       -3     
============================================
  Files           632      632              
  Lines         41473    41476       +3     
  Branches       5745     5746       +1     
============================================
- Hits          39929    39910      -19     
- Misses          617      619       +2     
- Partials        927      947      +20

Impacted Files	Coverage Δ	Complexity Δ
...activex/internal/operators/single/SingleDelay.java	100% <100%> (ø)	2 <1> (ø)	⬇️
src/main/java/io/reactivex/Single.java	100% <100%> (ø)	134 <3> (+2)	⬆️
.../operators/observable/ObservableFlatMapSingle.java	90.29% <0%> (-5.23%)	2% <0%> (ø)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...ternal/operators/observable/ObservablePublish.java	92.98% <0%> (-3.51%)	11% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.24% <0%> (-3.23%)	9% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	61.53% <0%> (-2.57%)	12% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	93.71% <0%> (-2.42%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	94.56% <0%> (-2.18%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableSkipLastTimed.java	95.91% <0%> (-2.05%)	2% <0%> (ø)
... and 26 more

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