Make ClosingFuture use PhantomReference instead of finalize().

Compare what 7c6b17c did for `FileBackedOutputStream`.

As in that CL, this CL adds `reachabilityFence` calls. I think that the calls are technically more necessary here than they were in `FileBackedOutputStream`, where we could probably [rely on `synchronized` to keep the object alive](https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/lang/ref/Reference.html#reachabilityFence(java.lang.Object)). Incidentally, they are equally necessary under the current `ClosingFuture` approach that uses `finalize()`. (So that gives some indication of how much they matter _in practice_: probably not much.)

I had a thought at one point that maybe `reachabilityFence` was necessary only in the _terminal_ operations: Since every `ClosingFuture` pipeline should end with a terminal operation, we should be able to rely on a `reachabilityFence` in that terminal operation to keep the chain alive. However, I now think it's best to put a fence in all operations:

- It's simpler.
- It behaves better for broken code that lacks a terminal operation: If not for fences on intermediate operations, then such code might see its `ClosingFuture` closed by GC while it's still operating on it. And while such code is buggy, I don't think we want to cause its intermediate operations to _throw_. By putting `reachabilityFence` calls in such operations, we keep the `ClosingFuture` alive until those operations complete, at which point it can leak "properly," getting closed with no harm to anyone except the resulting log message.

Finally: As a minor follow-up to 9744861, I arranged for the `Finalizer` thread to always have an unobfuscated _thread name_, even when the class name of `Finalizer` gets obfuscated.
RELNOTES=Migrated some prod classes from `finalize()` to `PhantomReference` in preparation for [the removal of finalization](https://openjdk.org/jeps/421). (7c6b17c)
PiperOrigin-RevId: 881753103

Author: cpovirk

android/guava/src/com/google/common/base/internal/Finalizer.java

@@ -72,7 +72,7 @@ public static void startFinalizer(
      */
 
     Finalizer finalizer = new Finalizer(finalizableReferenceClass, queue, frqReference);
-    String threadName = Finalizer.class.getName();
+    String threadName = "com.google.common.base.internal.Finalizer";
     Thread thread = null;
     if (bigThreadConstructor != null) {
       try {