Removing non SYSTEM user before running systemUserOnlyTest

Bug: 295446676
Test: atest UserManagerServiceTest -c
Change-Id: I6f536b7f5b85e4e884bd284b1848e43ac3c9029d
Merged-In: I6f536b7f5b85e4e884bd284b1848e43ac3c9029d
(cherry picked from commit b1bba3e9)

services/tests/mockingservicestests/src/com/android/server/pm/UserManagerServiceTest.java

@@ -323,6 +323,7 @@ public final class UserManagerServiceTest {
     @Test
     public void testGetBootUser_Headless_ThrowsIfOnlySystemUserExists() throws Exception {
         setSystemUserHeadless(true);
+        removeNonSystemUsers();
 
         assertThrows(UserManager.CheckedUserOperationException.class,
                 () -> mUmi.getBootUser(/* waitUntilSet= */ false));
@@ -346,6 +347,14 @@ public final class UserManagerServiceTest {
         return resultString.toString();
     }
 
+    private void removeNonSystemUsers() {
+        for (UserInfo user : mUms.getUsers(true)) {
+            if (!user.getUserHandle().isSystem()) {
+                mUms.removeUserInfo(user.id);
+            }
+        }
+    }
+
     private void mockCurrentUser(@UserIdInt int userId) {
         mockGetLocalService(ActivityManagerInternal.class, mActivityManagerInternal);