Removes type SYSTEM_ALERT_DIALOG from the A11yService warning dialog.

This type is unnecessary and causes unpleasant UX when the user attempts
to go home or open recent apps while the dialog is showing.

This does not affect the touch-consuming property of the dialog, so it
should still prevent interaction while another window is drawing over
it (FLAG_WINDOW_IS_OBSCURED/FLAG_WINDOW_IS_PARTIALLY_OBSCURED).

Bug: 336719951
Flag: android.view.accessibility.warning_use_default_dialog_type
Test: atest AccessibilityServiceWarningTest
Test: Observe that the dialog still works as expected, both when
      triggered by the Settings app and when triggered by the bespoke
      volume key shortcut editor.
Test: Observe that the dialog minimizes with the app window while
      switching apps.
Change-Id: Iaefd299b94df9cefd7fb5fbb4239c174d7bf7a63