Fix issue #7408647: Add getUserCount() API

Change-Id: Ib4e8e2300757e0941fae8fbc46c4323f5e2d7d6b

Author: Dianne Hackborn

api/17.txt

@@ -16619,6 +16619,7 @@ package android.os {
 
   public class UserManager {
     method public long getSerialNumberForUser(android.os.UserHandle);
+    method public int getUserCount();
     method public android.os.UserHandle getUserForSerialNumber(long);
     method public java.lang.String getUserName();
     method public boolean isUserAGoat();

api/current.txt

@@ -16619,6 +16619,7 @@ package android.os {
 
   public class UserManager {
     method public long getSerialNumberForUser(android.os.UserHandle);
+    method public int getUserCount();
     method public android.os.UserHandle getUserForSerialNumber(long);
     method public java.lang.String getUserName();
     method public boolean isUserAGoat();

core/java/android/os/UserManager.java

@@ -122,7 +122,7 @@ public boolean isUserRunningOrStopping(UserHandle user) {
      * @param userHandle the user handle of the user whose information is being requested.
      * @return the UserInfo object for a specific user.
      * @hide
-     * */
+     */
     public UserInfo getUserInfo(int userHandle) {
         try {
             return mService.getUserInfo(userHandle);
@@ -134,10 +134,11 @@ public UserInfo getUserInfo(int userHandle) {
 
     /**
      * Return the serial number for a user.  This is a device-unique
-     * number assigned to that user; if the user is deleted and new users
-     * created, the new users will not be given the same serial number.
+     * number assigned to that user; if the user is deleted and then a new
+     * user created, the new users will not be given the same serial number.
      * @param user The user whose serial number is to be retrieved.
-     * @return The serial number of the given user.
+     * @return The serial number of the given user; returns -1 if the
+     * given UserHandle does not exist.
      * @see #getUserForSerialNumber(long)
      */
     public long getSerialNumberForUser(UserHandle user) {
@@ -178,6 +179,14 @@ public UserInfo createUser(String name, int flags) {
         }
     }
 
+    /**
+     * Return the number of users currently created on the device.
+     */
+    public int getUserCount() {
+        List<UserInfo> users = getUsers();
+        return users != null ? users.size() : 1;
+    }
+
     /**
      * Returns information for all users on this device.
      * Requires {@link android.Manifest.permission#MANAGE_USERS} permission.