2.x: Allow @SchedulerSupport on constructors.

[vanniktech]

Motivation behind this is having a class which takes multiple constructor parameters where one can be a Scheduler with a default and a custom overload. In both cases I'd want to slam @SchedulerSupport on it so it can be handled properly by static analysis as well.

[codecov]

Codecov Report

Merging #5940 into 2.x will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5940      +/-   ##
============================================
+ Coverage     98.23%   98.24%   +<.01%     
- Complexity     6016     6020       +4     
============================================
  Files           656      656              
  Lines         44040    44040              
  Branches       6102     6102              
============================================
+ Hits          43262    43266       +4     
- Misses          232      235       +3     
+ Partials        546      539       -7

Impacted Files	Coverage Δ	Complexity Δ
...l/operators/observable/ObservableFlatMapMaybe.java	89.54% <0%> (-6.54%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-2.99%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-2.72%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	93.58% <0%> (-2.68%)	5% <0%> (ø)
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
...activex/internal/schedulers/ScheduledRunnable.java	98.07% <0%> (-1.93%)	29% <0%> (-1%)
... and 30 more

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[artem-zinnatullin]

lgtm

[artem-zinnatullin]

nit: maybe rename PR title to 2.x: Allow @SchedulerSupport on constructors.?

"Also" just distracts from the subject :)