Use SystemUIDialog.setWindowOnTop to make user-switching dialogs not steal focus

There is still the problem that the dialogs show up under QS, but
this change stops the dialog from stealing focus and thus deadlocking
the UI

Test: Add new user. Remove guest user. Note that you can then collapse
QS
Bug: 69591927

Change-Id: I489a094848c32d58e2bed4769bb249fbbef91ab5