Add @NonNull to the methods of Emitter

[hkurokawa]

This PR is a part of #4876

This PR adds @NonNull annotation to some methods of Emitter so that the users of RxJava migrating from 1.x to 2.x can easily know passing null to the methods will cause NPE.

[hkurokawa]

Sorry, I've just noticed there is #5151 related to this PR.

[codecov]

Codecov Report

Merging #5156 into 2.x will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5156      +/-   ##
============================================
+ Coverage     95.88%   95.91%   +0.02%     
+ Complexity     5606     5601       -5     
============================================
  Files           620      620              
  Lines         39856    39856              
  Branches       5604     5604              
============================================
+ Hits          38217    38227      +10     
+ Misses          654      651       -3     
+ Partials        985      978       -7

Impacted Files	Coverage Δ	Complexity Δ
...ternal/operators/flowable/FlowableSubscribeOn.java	90.9% <0%> (-5.46%)	2% <0%> (ø)
...rnal/subscribers/SinglePostCompleteSubscriber.java	94.87% <0%> (-5.13%)	14% <0%> (-1%)
...rnal/subscriptions/DeferredScalarSubscription.java	93.84% <0%> (-4.62%)	27% <0%> (-1%)
...internal/operators/completable/CompletableAmb.java	94.44% <0%> (-3.71%)	10% <0%> (-1%)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...x/internal/operators/flowable/FlowablePublish.java	91.92% <0%> (-3.14%)	10% <0%> (+1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	92.53% <0%> (-2.99%)	2% <0%> (ø)
...activex/internal/operators/single/SingleCache.java	97.05% <0%> (-2.95%)	23% <0%> (-1%)
...x/internal/operators/maybe/MaybeSwitchIfEmpty.java	97.22% <0%> (-2.78%)	2% <0%> (ø)
...activex/internal/disposables/DisposableHelper.java	98.14% <0%> (-1.86%)	27% <0%> (-1%)
... and 25 more

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[akarnokd]

Thanks @hkurokawa for the PR and @vanniktech for the review!

[hkurokawa]

@vanniktech @akarnokd Thank you for your prompt merge!