Add UserManagerInternal method to get the main user id. am: 051f571479 am: cc920b547c

Original change: https://android-review.googlesource.com/c/platform/frameworks/base/+/2378889

Change-Id: I9fb10b4098e33563b94222bba5aaa91520fd4acb
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>

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

@@ -6376,6 +6376,11 @@ public class UserManagerService extends IUserManager.Stub {
                 return userData != null && userData.getIgnorePrepareStorageErrors();
             }
         }
+
+        @Override
+        public @UserIdInt int getMainUserId() {
+            return UserHandle.USER_SYSTEM;
+        }
     }
 
     /**