Remove @Beta from FluentIterable.from(E[])

[cpovirk]

We already did this for from(Iterable).

As noted in #3425, we might have general reservations about committing to more FluentIterable methods now that Stream exists.

Aside from that, the only reservations I know of are:

• Arguably "from" was the wrong name for this, since it reflects changes to the backing array. (But this is equally true of the Iterable overload.) We might have preferred "for" (though that's a keyword, so we'd have to pick a longer name...).
• I think that it might not actually be truly 100% unmodifiable, since the returned iterator might be able to be cast to ListIterator, on which users could call set.

[kluever]

Apologies for the inexcusably long delay here. This is now fixed.