Hide isLinkedUser()

It's not the best API to use to determine anything. Defer to other
APIs such as UserManager.getUserRestrictions()

Bug: 8720520
Change-Id: Ie49589056ab52b4fdbcc736f8cdefadb8ba5d9d8

api/current.txt

@@ -17643,7 +17643,6 @@ package android.os {
     method public java.lang.String getUserName();
     method public android.os.Bundle getUserRestrictions();
     method public android.os.Bundle getUserRestrictions(android.os.UserHandle);
-    method public boolean isLinkedUser();
     method public boolean isUserAGoat();
     method public boolean isUserRunning(android.os.UserHandle);
     method public boolean isUserRunningOrStopping(android.os.UserHandle);

core/java/android/os/UserManager.java

@@ -202,6 +202,7 @@ public class UserManager {
      * Used to check if the user making this call is linked to another user. Linked users may have
      * a reduced number of available apps, app restrictions and account restrictions.
      * @return whether the user making this call is a linked user
+     * @hide
      */
     public boolean isLinkedUser() {
         try {