ICU-21480 Change ICU 64 draft readResolve APIs to @internal

See #1637
diff --git a/icu4j/main/classes/core/src/com/ibm/icu/text/DateIntervalFormat.java b/icu4j/main/classes/core/src/com/ibm/icu/text/DateIntervalFormat.java
index f417921..661fc79 100644
--- a/icu4j/main/classes/core/src/com/ibm/icu/text/DateIntervalFormat.java
+++ b/icu4j/main/classes/core/src/com/ibm/icu/text/DateIntervalFormat.java
@@ -377,8 +377,10 @@
         /**
          * serialization method resolve instances to the constant
          * DateIntervalFormat.SpanField values
-         * @draft ICU 64
+         * @internal
+         * @deprecated This API is ICU internal only.
          */
+        @Deprecated
         @Override
         protected Object readResolve() throws InvalidObjectException {
             if (this.getName().equals(DATE_INTERVAL_SPAN.getName()))
diff --git a/icu4j/main/classes/core/src/com/ibm/icu/text/ListFormatter.java b/icu4j/main/classes/core/src/com/ibm/icu/text/ListFormatter.java
index fee273c..6dfe743 100644
--- a/icu4j/main/classes/core/src/com/ibm/icu/text/ListFormatter.java
+++ b/icu4j/main/classes/core/src/com/ibm/icu/text/ListFormatter.java
@@ -171,8 +171,10 @@
         /**
          * Serialization method resolve instances to the constant Field values
          *
-         * @draft ICU 64
+         * @internal
+         * @deprecated This API is ICU internal only.
          */
+        @Deprecated
         @Override
         protected Object readResolve() throws InvalidObjectException {
             if (this.getName().equals(LITERAL.getName()))
diff --git a/icu4j/main/classes/core/src/com/ibm/icu/text/RelativeDateTimeFormatter.java b/icu4j/main/classes/core/src/com/ibm/icu/text/RelativeDateTimeFormatter.java
index 2a8b831..88ce0a4 100644
--- a/icu4j/main/classes/core/src/com/ibm/icu/text/RelativeDateTimeFormatter.java
+++ b/icu4j/main/classes/core/src/com/ibm/icu/text/RelativeDateTimeFormatter.java
@@ -437,8 +437,10 @@
         /**
          * Serizalization method resolve instances to the constant Field values
          *
-         * @draft ICU 64
+         * @internal
+         * @deprecated This API is ICU internal only.
          */
+        @Deprecated
         @Override
         protected Object readResolve() throws InvalidObjectException {
             if (this.getName().equals(LITERAL.getName()))