Tech debt cleanup: Consolidates A11yService warning dialog.

Both frameworks/base and the Settings app define almost-identical copies
of a warning dialog shown when enabling an accessibility service.
The frameworks/base version was used for contexts outside of Settings
(e.g. while editing the volume key shortcut after triggering it with
2+ features already enabled) while the Settings version was used in
the Settings app.

This change replaces version used in Settings with the version defined
in frameworks/base.

The warning dialog's functionality is tested in
AccessibilityServiceWarningTest in frameworks/base.

Feature flag:
`adb shell device_config override accessibility android.view.accessibility.deduplicate_accessibility_warning_dialog true`

Bug: 303511250
Test: atest AccessibilityServiceWarningTest
Test: existing A11ySettings robotests
Change-Id: Iad2f13040d53d82a4afb0353fae3ac355b5548d6