Creating a method to run in background the process to create a new user, and making the user check and change the UI in the main thread.
Also deleted the message of created user from the handler.
Bug:246906179
Test: Manual test and unit test fixed
Change-Id: Ifb3687937987f04165fd2e099d4334c7571bb034
Add user and delete, check on UI.
Bug: 188507135
Test: atest com.android.settings.users.UserSettingsComponentTest
Change-Id: I8aaa10f38a146cd3e869b5b5b35a93b451113e07