2.x: fix private field access, few generics problems

[akarnokd]

This PR fixes some private field accessor problems introduced by #5174 as well as a couple of generics warnings.

Note that in IntelliJ, there is a J2ME inspection for private access checks between parent and inner classes.

[codecov]

Codecov Report

Merging #5176 into 2.x will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5176      +/-   ##
============================================
+ Coverage     95.95%   95.96%   +0.01%     
+ Complexity     5660     5658       -2     
============================================
  Files           621      621              
  Lines         40247    40247              
  Branches       5610     5610              
============================================
+ Hits          38617    38625       +8     
  Misses          648      648              
+ Partials        982      974       -8

Impacted Files	Coverage Δ	Complexity Δ
...ternal/operators/completable/CompletableDelay.java	100% <ø> (ø)	2 <0> (ø)	❌
...s/observable/ObservableDelaySubscriptionOther.java	100% <ø> (ø)	2 <0> (ø)	❌
...ators/flowable/FlowableDelaySubscriptionOther.java	100% <ø> (ø)	2 <0> (ø)	❌
...activex/internal/operators/single/SingleDelay.java	100% <ø> (ø)	2 <0> (ø)	❌
...l/operators/completable/CompletableResumeNext.java	100% <ø> (ø)	2 <0> (ø)	❌
...rnal/operators/completable/CompletableTimeout.java	98% <ø> (ø)	2 <0> (ø)	❌
...tivex/internal/operators/single/SingleTimeout.java	96% <ø> (ø)	2 <0> (ø)	❌
...nternal/operators/completable/CompletablePeek.java	96.87% <100%> (ø)	2 <0> (ø)	❌
...nternal/operators/observable/ObservableReplay.java	97.18% <100%> (-0.57%)	18 <0> (ø)
...ex/internal/operators/flowable/FlowableReplay.java	91.04% <100%> (ø)	17 <0> (ø)	❌
... and 44 more

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[vanniktech]

PMD also has a check for those synthetic methods since 5.5.4