Use most of the main AbstractFuture implementation from J2KT and from GWT/J2CL.

This CL introduces a superclass, `AbstractFutureState`, following the pattern of [`AggregateFutureState`](https://github.com/google/guava/blob/master/guava/src/com/google/common/util/concurrent/AggregateFutureState.java). That superclass contains platform-specific operations.

Fixes #2934

RELNOTES=n/a
PiperOrigin-RevId: 729328833

Author: cpovirk

android/guava-testlib/src/com/google/common/testing/NullPointerTester.java

@@ -19,6 +19,8 @@
 import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.common.base.Preconditions.checkNotNull;
 import static java.util.Arrays.stream;
+import static java.util.Objects.requireNonNull;
+import static java.util.stream.Stream.concat;
 
 import com.google.common.annotations.GwtIncompatible;
 import com.google.common.annotations.J2ktIncompatible;
@@ -108,7 +110,9 @@ public NullPointerTester() {
      * versions of Java, and apparently Unsafe can cause SIGSEGV instead of NPE—almost as if it's
      * not safe.
      */
-    stream(AbstractFuture.class.getDeclaredMethods())
+    concat(
+            stream(AbstractFuture.class.getDeclaredMethods()),
+            stream(requireNonNull(AbstractFuture.class.getSuperclass()).getDeclaredMethods()))
         .filter(
             m ->
                 m.getName().equals("getDoneValue")

android/guava-tests/test/com/google/common/util/concurrent/AbstractFutureFallbackAtomicHelperTest.java

@@ -110,8 +110,8 @@ private void runTestMethod(ClassLoader classLoader) throws Exception {
   private void checkHelperVersion(ClassLoader classLoader, String expectedHelperClassName)
       throws Exception {
     // Make sure we are actually running with the expected helper implementation
-    Class<?> abstractFutureClass = classLoader.loadClass(AbstractFuture.class.getName());
-    Field helperField = abstractFutureClass.getDeclaredField("ATOMIC_HELPER");
+    Class<?> abstractFutureStateClass = classLoader.loadClass(AbstractFutureState.class.getName());
+    Field helperField = abstractFutureStateClass.getDeclaredField("ATOMIC_HELPER");
     helperField.setAccessible(true);
     assertEquals(expectedHelperClassName, helperField.get(null).getClass().getSimpleName());
   }

android/guava-tests/test/com/google/common/util/concurrent/AggregateFutureStateFallbackAtomicHelperTest.java

@@ -112,8 +112,8 @@ private void runTestMethod(ClassLoader classLoader) throws Exception {
   private void checkHelperVersion(ClassLoader classLoader, String expectedHelperClassName)
       throws Exception {
     // Make sure we are actually running with the expected helper implementation
-    Class<?> abstractFutureClass = classLoader.loadClass(AggregateFutureState.class.getName());
-    Field helperField = abstractFutureClass.getDeclaredField("ATOMIC_HELPER");
+    Class<?> abstractFutureStateClass = classLoader.loadClass(AggregateFutureState.class.getName());
+    Field helperField = abstractFutureStateClass.getDeclaredField("ATOMIC_HELPER");
     helperField.setAccessible(true);
     assertEquals(expectedHelperClassName, helperField.get(null).getClass().getSimpleName());
   }