2.x: Add missing license headers

[PhilGlass]

FixLicenseHeaders is currently failing when run locally (it's skipped on CI).

[codecov]

Codecov Report

Merging #5615 into 2.x will decrease coverage by 0.17%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##                2.x   #5615      +/-   ##
===========================================
- Coverage     96.27%   96.1%   -0.18%     
+ Complexity     5844    5841       -3     
===========================================
  Files           632     632              
  Lines         41473   41473              
  Branches       5745    5745              
===========================================
- Hits          39929   39857      -72     
- Misses          617     656      +39     
- Partials        927     960      +33

Impacted Files	Coverage Δ	Complexity Δ
...nternal/operators/observable/ObservableCreate.java	87.17% <0%> (-11.97%)	2% <0%> (ø)
...ex/internal/operators/flowable/FlowableCreate.java	90% <0%> (-7.75%)	6% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-3.81%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	91.79% <0%> (-3.74%)	2% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	92.98% <0%> (-3.51%)	11% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	92.75% <0%> (-3.39%)	2% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	60.28% <0%> (-2.84%)	32% <0%> (-2%)
...activex/internal/observers/QueueDrainObserver.java	61.53% <0%> (-2.57%)	12% <0%> (-1%)
...rnal/operators/observable/ObservableObserveOn.java	97.95% <0%> (-2.05%)	3% <0%> (ø)
...internal/operators/completable/CompletableAmb.java	98.3% <0%> (-1.7%)	11% <0%> (ø)
... and 20 more

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