Put "Add user" dialog within activity to capture focus

Dialogs are not a part of view hierarchy so if the dialog loses focus, it can be reached by traversing.

When it is placed within activity, then it can be accessed through traversing.

Bug: 376815882
Test: atest UserSettingsTest
Flag: android.multiuser.place_add_user_dialog_within_activity
Change-Id: I07a7eef061c894b81abfe6c8152c7b627b75bc4f