Exposed UserManager.getDisplayIdAssignedToUser as @TestApi

Test: m update-api # not used  yet, but will on CTS soon
Bug: 269805096

Change-Id: Id38438ae264989cda08c3fe97c602928bb3f11a5
diff --git a/core/api/test-current.txt b/core/api/test-current.txt
index def3fa9..8d55afb 100644
--- a/core/api/test-current.txt
+++ b/core/api/test-current.txt
@@ -2163,6 +2163,7 @@
     method @Nullable @RequiresPermission(anyOf={android.Manifest.permission.MANAGE_USERS, android.Manifest.permission.CREATE_USERS}) public android.content.pm.UserInfo createRestrictedProfile(@Nullable String);
     method @Nullable @RequiresPermission(anyOf={android.Manifest.permission.MANAGE_USERS, android.Manifest.permission.CREATE_USERS}) public android.content.pm.UserInfo createUser(@Nullable String, @NonNull String, int);
     method @NonNull @RequiresPermission(anyOf={android.Manifest.permission.MANAGE_USERS, android.Manifest.permission.CREATE_USERS}) public android.os.UserHandle getBootUser();
+    method public int getDisplayIdAssignedToUser();
     method @Nullable @RequiresPermission(anyOf={android.Manifest.permission.MANAGE_USERS, android.Manifest.permission.CREATE_USERS}) public java.util.Set<java.lang.String> getPreInstallableSystemPackages(@NonNull String);
     method @NonNull @RequiresPermission(anyOf={android.Manifest.permission.MANAGE_USERS, android.Manifest.permission.CREATE_USERS, android.Manifest.permission.QUERY_USERS}) public String getUserType();
     method @NonNull @RequiresPermission(anyOf={android.Manifest.permission.MANAGE_USERS, android.Manifest.permission.CREATE_USERS}) public java.util.List<android.content.pm.UserInfo> getUsers(boolean, boolean, boolean);
diff --git a/core/java/android/os/UserManager.java b/core/java/android/os/UserManager.java
index bbf7f81..bb35ca1 100644
--- a/core/java/android/os/UserManager.java
+++ b/core/java/android/os/UserManager.java
@@ -3056,6 +3056,7 @@
      *
      * @hide
      */
+    @TestApi
     public int getDisplayIdAssignedToUser() {
         try {
             return mService.getDisplayIdAssignedToUser();