Make more package-private classes final.

I skipped `ImmediateFuture`, since I don't want to deal with [the spying problems I'd encountered previously](https://github.com/google/guava/blob/4eb712cd7401086ebb48107de7515258ddc12c4b/guava/src/com/google/common/util/concurrent/ImmediateFuture.java#L29). I also ended up backing off from `SameThreadScheduledExecutorService`, which led to similar problems.

I also skipped [`Promise` in the web (GWT/J2CL) copy of `ListenableFuture`](https://github.com/google/guava/blob/4eb712cd7401086ebb48107de7515258ddc12c4b/guava-gwt/src-super/com/google/common/util/concurrent/super/com/google/common/util/concurrent/ListenableFuture.java#L83) because I don't know what the implications of `final` are there.

RELNOTES=n/a
PiperOrigin-RevId: 792194355

Author: cpovirk

android/guava-testlib/src/com/google/common/util/concurrent/testing/SameThreadScheduledExecutorService.java

@@ -45,6 +45,7 @@
  * @author Zach van Schouwen
  */
 @GwtIncompatible
+// TODO(cpovirk): Make this final (but that may break Mockito spy calls).
 class SameThreadScheduledExecutorService extends AbstractExecutorService
     implements ListeningScheduledExecutorService {
 

android/guava/src/com/google/common/cache/LocalCache.java

@@ -103,7 +103,7 @@
 })
 @GwtCompatible
 @NullUnmarked // TODO(cpovirk): Annotate for nullness.
-class LocalCache<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V> {
+final class LocalCache<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V> {
 
   /*
    * The basic strategy is to subdivide the table among Segments, each of which itself is a

android/guava/src/com/google/common/io/AppendableWriter.java

@@ -34,7 +34,7 @@
  */
 @J2ktIncompatible
 @GwtIncompatible
-class AppendableWriter extends Writer {
+final class AppendableWriter extends Writer {
   private final Appendable target;
   private boolean closed;
 

android/guava/src/com/google/common/io/MultiReader.java

@@ -32,7 +32,7 @@
  */
 @J2ktIncompatible
 @GwtIncompatible
-class MultiReader extends Reader {
+final class MultiReader extends Reader {
   private final Iterator<? extends CharSource> it;
   private @Nullable Reader current;
 

guava-testlib/src/com/google/common/util/concurrent/testing/SameThreadScheduledExecutorService.java

@@ -45,6 +45,7 @@
  * @author Zach van Schouwen
  */
 @GwtIncompatible
+// TODO(cpovirk): Make this final (but that may break Mockito spy calls).
 class SameThreadScheduledExecutorService extends AbstractExecutorService
     implements ListeningScheduledExecutorService {
 

guava/src/com/google/common/cache/LocalCache.java

@@ -107,7 +107,7 @@
 })
 @GwtCompatible
 @NullUnmarked // TODO(cpovirk): Annotate for nullness.
-class LocalCache<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V> {
+final class LocalCache<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V> {
 
   /*
    * The basic strategy is to subdivide the table among Segments, each of which itself is a

guava/src/com/google/common/io/AppendableWriter.java

@@ -34,7 +34,7 @@
  */
 @J2ktIncompatible
 @GwtIncompatible
-class AppendableWriter extends Writer {
+final class AppendableWriter extends Writer {
   private final Appendable target;
   private boolean closed;
 

guava/src/com/google/common/io/MultiReader.java

@@ -32,7 +32,7 @@
  */
 @J2ktIncompatible
 @GwtIncompatible
-class MultiReader extends Reader {
+final class MultiReader extends Reader {
   private final Iterator<? extends CharSource> it;
   private @Nullable Reader current;