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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