Don't use ImmutableSet.of() for the keySet of ImmutableMap.of().

The keySet of the empty map will still be a singleton (since the empty map is a singleton and it caches keySet), it'll just be a different singleton from ImmutableSet.of().  This ensures that the only implementation of ImmutableSet retained by usages of ImmutableMap are its own keySet implementations.

Effects on ImmutableMapProGuard ([]38966 bytes => 37886 bytes
37 classes => 36 classes
231 methods => 219 methods

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=147078624

guava/src/com/google/common/collect/ImmutableMap.java

@@ -669,7 +669,7 @@ public ImmutableSet<K> keySet() {
   }
 
   ImmutableSet<K> createKeySet() {
-    return isEmpty() ? ImmutableSet.<K>of() : new ImmutableMapKeySet<K, V>(this);
+    return new ImmutableMapKeySet<K, V>(this);
   }
 
   UnmodifiableIterator<K> keyIterator() {