2.x: Fix Flowable.window(Publisher|Callable) upstream handling

[akarnokd]

This PR fixes the upstream handling in Flowable.window(Publisher) and Flowable.window(Callable<Publisher>) operators to make sure if both the main output and the inner windows have been cancelled, the upstream is also cancelled.

Related: #5887, #5881.

In addition, the Jacoco code coverage has been updated to 0.8.0 mainly due to the automatic filtering of difficult-to-impossible code paths.

[codecov]

Codecov Report

Merging #5888 into 2.x will increase coverage by 0.06%.
The diff coverage is 98.65%.

@@             Coverage Diff              @@
##                2.x    #5888      +/-   ##
============================================
+ Coverage     97.49%   97.55%   +0.06%     
+ Complexity     5946     5941       -5     
============================================
  Files           655      655              
  Lines         43900    43862      -38     
  Branches       6108     6100       -8     
============================================
- Hits          42799    42789      -10     
+ Misses          339      328      -11     
+ Partials        762      745      -17

Impacted Files	Coverage Δ	Complexity Δ
...ators/flowable/FlowableWindowBoundarySupplier.java	98.74% <98.33%> (+7.23%)	2 <1> (ø)	⬇️
...nal/operators/flowable/FlowableWindowBoundary.java	99.28% <99.02%> (+7.55%)	2 <1> (ø)	⬇️
...nternal/operators/observable/ObservableCreate.java	93.91% <0%> (-5.24%)	2% <0%> (ø)
...in/java/io/reactivex/subjects/BehaviorSubject.java	95.74% <0%> (-2.67%)	54% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	96.86% <0%> (-2.25%)	60% <0%> (ø)
...perators/mixed/ObservableConcatMapCompletable.java	98.23% <0%> (-1.77%)	2% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	96.66% <0%> (-1.67%)	2% <0%> (ø)
...vex/internal/operators/flowable/FlowableCache.java	92.61% <0%> (-1.35%)	7% <0%> (ø)
...ex/internal/operators/flowable/FlowableCreate.java	96.1% <0%> (-1.32%)	6% <0%> (ø)
...perators/mixed/ObservableSwitchMapCompletable.java	98.93% <0%> (-1.07%)	2% <0%> (ø)
... and 42 more

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