2.x: fix window(time, size) not completing windows on timeout

[akarnokd]

This PR fixes the operators Flowable.window(time, size) and Observable.window(time, size) to complete the current window if the time elapses before the size limit is reached.

Reported in #5104.

[codecov]

Codecov Report

Merging #5106 into 2.x will increase coverage by 0.06%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##               2.x    #5106      +/-   ##
===========================================
+ Coverage     95.6%   95.67%   +0.06%     
+ Complexity    5558     5556       -2     
===========================================
  Files          615      615              
  Lines        39703    39705       +2     
  Branches      5583     5583              
===========================================
+ Hits         37959    37986      +27     
+ Misses         758      745      -13     
+ Partials       986      974      -12

Impacted Files	Coverage Δ	Complexity Δ
...ternal/operators/flowable/FlowableWindowTimed.java	83.86% <100%> (+0.26%)	4 <ø> (ø)	❌
...al/operators/observable/ObservableWindowTimed.java	87.84% <100%> (+0.03%)	4 <ø> (ø)	❌
...a/io/reactivex/processors/SerializedProcessor.java	92.55% <ø> (-5.32%)	26% <ø> (-2%)
...rnal/operators/completable/CompletableTimeout.java	92.3% <ø> (-5.13%)	2% <ø> (ø)
...erators/completable/CompletableConcatIterable.java	95.91% <ø> (-4.09%)	2% <ø> (ø)
...operators/observable/ObservableConcatMapEager.java	97.29% <ø> (-2.17%)	2% <ø> (ø)
...reactivex/internal/operators/maybe/MaybeUsing.java	93.93% <ø> (-2.03%)	4% <ø> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <ø> (-2%)	34% <ø> (-1%)
...activex/internal/disposables/DisposableHelper.java	98.14% <ø> (-1.86%)	27% <ø> (-1%)
...internal/operators/completable/CompletableAmb.java	96.29% <ø> (-1.86%)	10% <ø> (-1%)
... and 37 more

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