Updates documentation for MessagingStyle constructor

MessagingStyle expects the field userReplyName to be non-null, but the
documentation doesn't describe it as such. This updates the documentation
to say the field is required, and adds a NonNull annotation.

This has no behavior changes.

BUG:31747744
Change-Id: If832d059c276e856fba366dabfa8a5821bb63054
diff --git a/core/java/android/app/Notification.java b/core/java/android/app/Notification.java
index 981a055..1103d9e 100644
--- a/core/java/android/app/Notification.java
+++ b/core/java/android/app/Notification.java
@@ -19,6 +19,7 @@
 import android.annotation.ColorInt;
 import android.annotation.DrawableRes;
 import android.annotation.IntDef;
+import android.annotation.NonNull;
 import android.annotation.SdkConstant;
 import android.annotation.SdkConstant.SdkConstantType;
 import android.annotation.SystemApi;
@@ -4649,12 +4650,12 @@
         }
 
         /**
-         * @param userDisplayName the name to be displayed for any replies sent by the user before the
-         * posting app reposts the notification with those messages after they've been actually
-         * sent and in previous messages sent by the user added in
+         * @param userDisplayName Required - the name to be displayed for any replies sent by the
+         * user before the posting app reposts the notification with those messages after they've
+         * been actually sent and in previous messages sent by the user added in
          * {@link #addMessage(Notification.MessagingStyle.Message)}
          */
-        public MessagingStyle(CharSequence userDisplayName) {
+        public MessagingStyle(@NonNull CharSequence userDisplayName) {
             mUserDisplayName = userDisplayName;
         }