cleanup(expressive): Use AndroidX dialog for the a11yservice warning

The Settings app theme only applies to AndroidX dialogs so this is
necessary in order to use correct colors for this dialog.

Fix: 411506415
Test: atest ToggleAccessibilityServicePreferenceFragmentTest
Test: use the dialog to enable, deny, and uninstall a service
Test: observe dialog theme looks correct; see screenshots in bug
Flag: EXEMPT bugfix via refactor
Change-Id: If13e6256dbcffb2f5392b218bf01164ef7733816