3.x: Add missing NonNull annotations to the standard methods

[akarnokd]

Including the return type, parameters and type argument usages on lambdas.

• Flowable
• Observable
• Maybe
• Single
• Completable
• ParallelFlowable

Please do them in separate PRs.

[akarnokd]

We can annotate the type arguments on our base interfaces and those should be enough for most of the usage places, however, some 3rd party interfaces need explicit annotations:

• Publisher<@NonNull
• Iterable<@NonNull
• Stream<@NonNull

The original pattern could be Type<AnotherType or Type<? extends locations.