Add FunctionalInterface annotations.

[He-Pin]

Resolves #6768

[codecov]

Codecov Report

Merging #6773 into 3.x will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##               3.x    #6773      +/-   ##
===========================================
+ Coverage     98.2%   98.21%   +<.01%     
- Complexity    6183     6190       +7     
===========================================
  Files          681      681              
  Lines        44776    44776              
  Branches      6164     6164              
===========================================
+ Hits         43973    43977       +4     
+ Misses         280      279       -1     
+ Partials       523      520       -3

Impacted Files	Coverage Δ	Complexity Δ
...l/operators/observable/ObservableFlatMapMaybe.java	93.66% <0%> (-2.82%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	90.47% <0%> (-2.39%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	90.1% <0%> (-1.05%)	2% <0%> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.05% <0%> (-0.95%)	2% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	97.43% <0%> (-0.86%)	2% <0%> (ø)
...internal/operators/flowable/FlowableObserveOn.java	96.65% <0%> (-0.84%)	3% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	95.83% <0%> (-0.84%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelSortedJoin.java	94.16% <0%> (-0.73%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	93.6% <0%> (-0.59%)	2% <0%> (ø)
...ternal/operators/flowable/FlowableWindowTimed.java	96.62% <0%> (-0.57%)	5% <0%> (ø)
... and 14 more

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