3.x: Fix ExecutorScheduler initializing Schedulers prematurely

[akarnokd]

ExecutorScheduler had a static field referencing Schedulers thus when using RxJavaPlugins.createExecutorScheduler, it would still initialize the standard schedulers despite the intention of the create method. Using the static inner holder class idiom can prevent this, similar to how Schedulers does it.

Related: #7306 (comment)

[codecov]

Codecov Report

Merging #7323 (fdf8010) into 3.x (804b8ef) will increase coverage by 0.01%.
The diff coverage is 75.00%.

❗ Current head fdf8010 differs from pull request most recent head 5b664b3. Consider uploading reports for the commit 5b664b3 to get more accurate results

@@             Coverage Diff              @@
##                3.x    #7323      +/-   ##
============================================
+ Coverage     99.53%   99.54%   +0.01%     
+ Complexity     6784     6783       -1     
============================================
  Files           751      751              
  Lines         47490    47491       +1     
  Branches       6378     6378              
============================================
+ Hits          47269    47277       +8     
+ Misses          102       96       -6     
+ Partials        119      118       -1     

Impacted Files	Coverage Δ
...rxjava3/internal/schedulers/ExecutorScheduler.java	99.56% <75.00%> (-0.44%)	⬇️
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0.00%> (-4.66%)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	94.44% <0.00%> (-3.97%)	⬇️
...nternal/operators/parallel/ParallelReduceFull.java	93.06% <0.00%> (-1.99%)	⬇️
...ternal/operators/completable/CompletableMerge.java	97.29% <0.00%> (-1.36%)	⬇️
.../operators/observable/ObservableCombineLatest.java	99.38% <0.00%> (-0.62%)	⬇️
.../reactivex/rxjava3/observers/BaseTestConsumer.java	99.46% <0.00%> (-0.54%)	⬇️
...a3/internal/operators/flowable/FlowableCreate.java	99.02% <0.00%> (-0.33%)	⬇️
...3/internal/operators/flowable/FlowableGroupBy.java	84.73% <0.00%> (-0.30%)	⬇️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	97.40% <0.00%> (ø)
... and 10 more

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