Add UserManagerInternal method to get the main user id.

This adds a method to get the id of the main user on the device.

Currently this always returns the id of the system user.

Bug: 256624031
Test: atest VpnManagerServiceTest (with topic)
Merged-In: Ib36f3b372f9bf33cbb097c4af63eb43515ac835b
Change-Id: I4aa0feb68083460c6aef917337e8bfc09a6788d4

services/core/java/com/android/server/pm/UserManagerInternal.java

@@ -305,4 +305,10 @@ public abstract class UserManagerInternal {
      * for users that already existed on-disk from an older version of Android.
      */
     public abstract boolean shouldIgnorePrepareStorageErrors(int userId);
+
+    /**
+     * Returns the user id of the main user, or {@link android.os.UserHandle#USER_NULL} if there is
+     * no main user.
+     */
+    public abstract @UserIdInt int getMainUserId();
 }

services/core/java/com/android/server/pm/UserManagerService.java

@@ -6359,6 +6359,11 @@ public class UserManagerService extends IUserManager.Stub {
                 return userData != null && userData.getIgnorePrepareStorageErrors();
             }
         }
+
+        @Override
+        public @UserIdInt int getMainUserId() {
+            return UserHandle.USER_SYSTEM;
+        }
     }
 
     /**