Make FileBackedOutputStream use PhantomReference instead of finalize().

[copybara-service]

Make FileBackedOutputStream use PhantomReference instead of finalize().

This generally makes it more resilient and future-proof, but we still don't recommend relying on GC for object cleanup, in part because any small change (like this one) could upset a delicate balance somewhere.

Also, while I'm messing with reachabilityFence again: Remove a confused comment from 7eba58f in GcFinalizationTest: Once we use reachabilityFence directly, we'll eliminate the helper method, and http://errorprone.info/bugpattern/ReachabilityFenceUsage will remain happy because the call to reachabilityFence will appear directly inside a finally block. (I'm still not sure that finally is necessary there, but it's easy, and it's probably reasonable as a default practice.)

(I'm also not sure if finally is necessary for the new usages of reachabilityFence in FileBackedOutputStream. I guess the fear would be that we'd allow the FileOutputStream to be closed while it's in the process of preparing to throw an exception, and the close call could clear something that the exception-construction code wants to read? I do suspect that the new reachabilityFence calls are a good idea to have somewhere, whether in a finally block or not, to ensure that background cleanup doesn't reset a FileBackedOutputStream during a call to a method like write (which might then open a new file) or close (which might then not see an exception because the background reset sees it instead). Or maybe reachabilityFence isn't necessary because our methods are synchronized. (synchronized should at least mean that we don't need the fence in a finally block, at least not for the reasons that I mentioned above: It won't be possible to reset the stream until write or close exits the critical area.) But presumably it won't hurt anything.)

RELNOTES=n/a