2.x: A.flatMapB to eagerly check for cancellations before subscribing

[akarnokd]

The flatMap implementations always subscribed to the generated inner source even if there was an asynchronous cancel while the function was running. With typical functions, there is only a really tiny window inside function but some users tend to block/sleep in the function and when that returns, the flatMap operator is already cancelled.

If the generated inner source emitted an error disregarding its own cancellation signal (the error() operators do this), those errors end up in the RxJavaPlugins.onError and crash the app (on Android).

This PR adjusts the flatMap implementations to check for the disposed/cancelled state before subscribing to the inner source. For Observable and Flowable, this has practically no extra overhead as the add/remove already checks for the terminal state and can return a boolean for it. The rest require an explicit isDisposed() check.

[codecov-io]

Current coverage is 95.58% (diff: 100%)

Merging #4992 into 2.x will increase coverage by 0.05%

@@                2.x      #4992   diff @@
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[JakeWharton]

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