Remove @Beta from Futures.transform, catching, and *Async variants

[cpovirk]

These methods are very heavily used internally. They're probably less so externally, but that may change as AndroidX exposes ListenableFuture in more APIs. Plus, Dagger Producers uses these methods.

Blockers:

• likely change to make catching not catch the CancellationException from cancellation (internal bug 27587642)
• possible changes to interruption behavior (https://github.com/google/guava/issues/1989)

[cpovirk]

We could even try to change the parameter type to the JDK Function someday (using a trick to temporarily maintain binary compatibility), but I'm not sure we'd do it.

[cpovirk]

https://github.com/google/guava/issues/3620 mentions the possibility of changing the return type of these methods (and others). But we could still do that after removing @Beta as long as we don't "just change" it: We'd have to inject bridge methods with the old signatures, as we already do for some other cases of signature changes.

[cpovirk]

Anyway, my thinking lately has been that the possible interruption change doesn't necessarily block this, either, but that the possible CancellationException change for catching probably does.

I'm not entirely sure why I feel that way: Both are changes to behavior but not to mention signatures, and both are likely to have negative results for a very small percentage of callers. Maybe it's the fact that the CancellationException behavior is actually documented?

I'm going to avoid making a snap judgment after not having thought about this in a long time. But maybe I can talk myself into this. Worst case, there are a number of other methods in Futures that we probably can easily remove @Beta from.

[kluever]

This is now fixed.