2.x: Small cleanups in the Scheduler class

[akarnokd]

• Remove @NonNull from a primitive type
• Make the package-private PeriodicDirectTask final
• Add @NonNull to other fields

[codecov]

Codecov Report

Merging #5985 into 2.x will decrease coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5985      +/-   ##
============================================
- Coverage     98.27%   98.26%   -0.02%     
+ Complexity     6051     6050       -1     
============================================
  Files           656      656              
  Lines         44078    44078              
  Branches       6118     6118              
============================================
- Hits          43318    43312       -6     
- Misses          226      231       +5     
- Partials        534      535       +1

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Scheduler.java	100% <100%> (ø)	11 <0> (ø)	⬇️
...nternal/operators/observable/ObservableCreate.java	93.04% <0%> (-5.22%)	2% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	96.52% <0%> (-3.48%)	55% <0%> (-3%)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	94.56% <0%> (-2.18%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	92.27% <0%> (-1.45%)	2% <0%> (ø)
...java/io/reactivex/processors/UnicastProcessor.java	98.8% <0%> (-1.2%)	67% <0%> (-1%)
...ivex/internal/operators/maybe/MaybeMergeArray.java	96.62% <0%> (-1.13%)	6% <0%> (ø)
...perators/mixed/ObservableConcatMapCompletable.java	99.24% <0%> (-0.76%)	3% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-0.75%)	2% <0%> (ø)
... and 18 more

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