2.x: prevent tasks to self interrupt on the standard schedulers

[akarnokd]

Task wrappers of the various schedulers and modes (direct & worker) were able to get cancelled via interruption from the same thread they were running.

Related #5203.

[codecov]

Codecov Report

Merging #5207 into 2.x will increase coverage by 0.05%.
The diff coverage is 98.93%.

@@             Coverage Diff             @@
##               2.x    #5207      +/-   ##
===========================================
+ Coverage     95.9%   95.95%   +0.05%     
- Complexity    5678     5706      +28     
===========================================
  Files          621      624       +3     
  Lines        40611    40677      +66     
  Branches      5632     5644      +12     
===========================================
+ Hits         38947    39032      +85     
+ Misses         673      660      -13     
+ Partials       991      985       -6

Impacted Files	Coverage Δ	Complexity Δ
...activex/internal/schedulers/ExecutorScheduler.java	94.59% <100%> (-1.89%)	9 <0> (ø)
...reactivex/internal/schedulers/NewThreadWorker.java	100% <100%> (ø)	17 <5> (+2)	⬆️
...ternal/schedulers/ScheduledDirectPeriodicTask.java	100% <100%> (ø)	2 <2> (?)
...reactivex/internal/schedulers/SingleScheduler.java	95.77% <100%> (+0.12%)	13 <3> (ø)	⬇️
...tivex/internal/schedulers/ScheduledDirectTask.java	100% <100%> (ø)	2 <2> (?)
...ctivex/internal/schedulers/AbstractDirectTask.java	100% <100%> (ø)	16 <16> (?)
...activex/internal/schedulers/ScheduledRunnable.java	95.74% <100%> (+2.41%)	27 <7> (+3)	⬆️
src/main/java/io/reactivex/Scheduler.java	98.83% <91.66%> (-1.17%)	11 <0> (ø)
...al/operators/observable/ObservableSampleTimed.java	88.33% <0%> (-8.34%)	3% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	91.22% <0%> (-5.27%)	9% <0%> (-2%)
... and 44 more

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[vanniktech]

👍 didn't quite understand everything but the tests seem good to me