3.x: Fix Observable.window (size & time) cancellation and abandonment

[akarnokd]

This PR fixes the Observable.window operator (with size and time boundaries) so that

• cancelling the inner windows allows cancelling the upstream once neither the main output nor other windows are being consumed further,
• ignoring a window still allows cancelling the upstream.

Follow-up to #6758

The boundary and start-stop publisher boundarywill be updated in subsequent PRs.

In addition, the previous WindowSubscribeIntercept has been renamed to match the reactive type naming as well as removed the unnecessary serialization of the emission of window processors.

[codecov]

Codecov Report

Merging #6761 into 3.x will increase coverage by <.01%.
The diff coverage is 95.6%.

@@ Coverage Diff @@ ## 3.x #6761 +/- ## ============================================ + Coverage 98.18% 98.19% +<.01%  - Complexity 6193 6196 +3  ============================================ Files 678 679 +1 Lines 44580 44552 -28 Branches 6152 6145 -7 ============================================ - Hits 43773 43746 -27  - Misses 285 286 +1  + Partials 522 520 -2

Impacted Files	Coverage Δ	Complexity Δ
...ors/flowable/FlowableWindowSubscribeIntercept.java	87.5% <100%> (ø)	4 <1> (?)
...ternal/operators/flowable/FlowableWindowTimed.java	96.62% <100%> (-0.01%)	5 <0> (ø)
...nternal/operators/observable/ObservableWindow.java	100% <100%> (ø)	3 <0> (ø)	⬇️
...a3/internal/operators/flowable/FlowableWindow.java	89.8% <100%> (ø)	4 <0> (ø)	⬇️
...observable/ObservableWindowSubscribeIntercept.java	87.5% <87.5%> (ø)	4 <4> (?)
...al/operators/observable/ObservableWindowTimed.java	96.12% <95.37%> (+4.86%)	4 <0> (ø)	⬇️
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...eactivex/rxjava3/processors/BehaviorProcessor.java	96.58% <0%> (-2.44%)	51% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	94.27% <0%> (-2.09%)	2% <0%> (ø)
...java3/internal/operators/single/SingleTimeout.java	98.33% <0%> (-1.67%)	2% <0%> (ø)
... and 18 more

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