3.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.

The same fix for 2.x will be posted separately.

Fixes: #6647

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (3.x@9a36930). Click here to learn what that means.
The diff coverage is 100%.

@@          Coverage Diff          @@
##             3.x   #6648   +/-   ##
=====================================
  Coverage       ?   98.1%           
  Complexity     ?    6191           
=====================================
  Files          ?     678           
  Lines          ?   44722           
  Branches       ?    6176           
=====================================
  Hits           ?   43875           
  Misses         ?     295           
  Partials       ?     552

Impacted Files	Coverage Δ	Complexity Δ
.../operators/observable/ObservableTakeLastTimed.java	98.68% <100%> (ø)	2 <0> (?)

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