2.x: Fix takeLast(time) last events time window calculation.

[akarnokd]

The logic inside the Observable.takeLast(time) was not using a fixed timestamp to compare against but one that could change between calls, resulting in items wrongly skipped from the accumulated buffer. The PR makes this timestamp limit fixed outside the drain loop.

The Flowable variant did not have the issue but both received the unit test verifying the correct behavior.

Fixes: #6647

[codecov]

Codecov Report

Merging #6653 into 2.x will decrease coverage by 0.06%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6653      +/-   ##
============================================
- Coverage     98.24%   98.18%   -0.07%     
- Complexity     6344     6345       +1     
============================================
  Files           677      677              
  Lines         45550    45551       +1     
  Branches       6332     6332              
============================================
- Hits          44752    44724      -28     
- Misses          248      262      +14     
- Partials        550      565      +15

Impacted Files	Coverage Δ	Complexity Δ
.../operators/observable/ObservableTakeLastTimed.java	98.68% <100%> (+0.01%)	2 <0> (ø)	⬇️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	91.3% <0%> (-5.32%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	90.29% <0%> (-5.23%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	96.49% <0%> (-3.51%)	12% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	95.96% <0%> (-3.14%)	59% <0%> (-1%)
...perators/single/SingleFlatMapIterableFlowable.java	95.83% <0%> (-2.5%)	2% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...activex/internal/schedulers/ScheduledRunnable.java	98.07% <0%> (-1.93%)	29% <0%> (-1%)
...tivex/internal/operators/single/SingleTimeout.java	98.33% <0%> (-1.67%)	2% <0%> (ø)
... and 17 more

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