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 the frameworks/base implementation with the more
full-featured Settings implementation.
The other change in this topic in packages/apps/Settings changes
Settings to use this consolidated single version.

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

Bug: 303511250
Test: Use this dialog in Accessibility Settings
Test: Use this dialog in the a11y volume-key shortcut editor
Test: atest AccessibilityServiceWarningTest
Test: atest AccessibilityShortcutChooserActivityTest
Change-Id: I6e1acf7cecfaa0dce422f7dfac3f270b9902dfe9