Combine immutability type fixes. (#3)

* Update Schema.java

The type sets are final mutable types, but with immutable implementations. This doesn't communicate the desired semantic guarantees of the immutable implementation.

* Remove unneeded collection import.

sdks/java/core/src/main/java/org/apache/beam/sdk/schemas/Schema.java

@@ -28,7 +28,6 @@
 import java.util.Arrays;
 import java.util.List;
 import java.util.Objects;
-import java.util.Set;
 import java.util.stream.Collector;
 import java.util.stream.Collectors;
 import javax.annotation.Nullable;
@@ -222,13 +221,13 @@ public enum TypeName {
     MAP,
     ROW;    // The field is itself a nested row.
 
-    public static final Set<TypeName> NUMERIC_TYPES = ImmutableSet.of(
+    public static final ImmutableSet<TypeName> NUMERIC_TYPES = ImmutableSet.of(
         BYTE, INT16, INT32, INT64, DECIMAL, FLOAT, DOUBLE);
-    public static final Set<TypeName> STRING_TYPES = ImmutableSet.of(STRING);
-    public static final Set<TypeName> DATE_TYPES = ImmutableSet.of(DATETIME);
-    public static final Set<TypeName> COLLECTION_TYPES = ImmutableSet.of(ARRAY);
-    public static final Set<TypeName> MAP_TYPES = ImmutableSet.of(MAP);
-    public static final Set<TypeName> COMPOSITE_TYPES = ImmutableSet.of(ROW);
+    public static final ImmutableSet<TypeName> STRING_TYPES = ImmutableSet.of(STRING);
+    public static final ImmutableSet<TypeName> DATE_TYPES = ImmutableSet.of(DATETIME);
+    public static final ImmutableSet<TypeName> COLLECTION_TYPES = ImmutableSet.of(ARRAY);
+    public static final ImmutableSet<TypeName> MAP_TYPES = ImmutableSet.of(MAP);
+    public static final ImmutableSet<TypeName> COMPOSITE_TYPES = ImmutableSet.of(ROW);
 
     public boolean isPrimitiveType() {
       return !isCollectionType() && !isMapType() && !isCompositeType();