Remove dependency of Schedulers from ObservableRefCount

[mycallmax]

Resolves #6451.

In the constructor of ObservableRefCount that takes ConnectableObservable<T> source as the argument, we set timeout to 0L. In that specific use case of ObservableRefCount, scheduler is never needed. It's only referenced in cancel() method but if timeout is 0, it won't be triggered at all because there is early return. This commit removes the need to depend on Schedulers.trampoline() and instead passes null to be scheduler when the ref count is not time-based. Similarly, applies the same change to FlowableRefCount.

The reasons for this change are the following:

1. In projects that don't depend on Schedulers class, if there is no reference to Schedulers, the whole Schedulers can be stripped out of the library after optimizations (e.g., proguard). With constructor that references Schedulers, the optimizer can't properly strip it out. In our quick test of our Android app, we were able to reduce the RxJava library size dependency from 51KB to 37KB (after optimization but before compression) by simply avoiding access to Schedulers in ObservableRefCount.
2. In terms of modularity, ObservableRefCount is just an operator so it by itself should probably not have dependency on what available pool of schedulers (Schedulers) there are. It should just know that there is some Scheduler that could be passed to ObservableRefCount when ObservableRefCount needs it.

[codecov]

Codecov Report

Merging #6452 into 2.x will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6452      +/-   ##
============================================
+ Coverage     98.27%   98.31%   +0.03%     
- Complexity     6296     6298       +2     
============================================
  Files           675      675              
  Lines         45173    45173              
  Branches       6246     6246              
============================================
+ Hits          44393    44410      +17     
+ Misses          241      238       -3     
+ Partials        539      525      -14

Impacted Files	Coverage Δ	Complexity Δ
...ernal/operators/observable/ObservableRefCount.java	100% <100%> (ø)	28 <1> (ø)	⬇️
.../internal/operators/flowable/FlowableRefCount.java	100% <100%> (ø)	28 <1> (ø)	⬇️
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	90.82% <0%> (-3.87%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...ava/io/reactivex/processors/BehaviorProcessor.java	96.86% <0%> (-2.25%)	60% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	92.15% <0%> (-1.97%)	2% <0%> (ø)
...operators/observable/ObservableMergeWithMaybe.java	99.1% <0%> (-0.9%)	2% <0%> (ø)
.../operators/maybe/MaybeFlatMapIterableFlowable.java	97.54% <0%> (-0.82%)	2% <0%> (ø)
...ex/internal/operators/flowable/FlowableCreate.java	97.09% <0%> (ø)	6% <0%> (ø)	⬇️
... and 16 more

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[mycallmax]

Thanks for the review and the approval!