3.x: Make DisposableContainer public API

[akarnokd]

Make the internal interface DisposableContainer public as it can be one safely.

(I've seen some 3rd party usage in custom containers, this will make them now legit on 3.x.)

Edit
Also should resolve the OSGi private reference warning.

Resolves #6742

[codecov]

Codecov Report

Merging #6745 into 3.x will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6745      +/-   ##
============================================
- Coverage     98.12%   98.08%   -0.05%     
+ Complexity     6189     6187       -2     
============================================
  Files           677      677              
  Lines         44663    44663              
  Branches       6171     6171              
============================================
- Hits          43825    43806      -19     
- Misses          299      311      +12     
- Partials        539      546       +7

Impacted Files	Coverage Δ	Complexity Δ
.../internal/disposables/ListCompositeDisposable.java	98% <ø> (-2%)	34 <0> (-1)
...rxjava3/internal/schedulers/ScheduledRunnable.java	100% <ø> (ø)	30 <0> (ø)	⬇️
...tivex/rxjava3/disposables/CompositeDisposable.java	100% <ø> (ø)	40 <0> (ø)	⬇️
...x/rxjava3/internal/schedulers/NewThreadWorker.java	94.91% <ø> (ø)	19 <0> (ø)	⬇️
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...l/operators/observable/ObservableFlatMapMaybe.java	88.73% <0%> (-3.53%)	2% <0%> (ø)
...ctivex/rxjava3/internal/util/QueueDrainHelper.java	97.22% <0%> (-2.78%)	56% <0%> (-2%)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	95.94% <0%> (-1.36%)	2% <0%> (ø)
... and 20 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 445def8...319d1ef. Read the comment docs.