Use A11yManagerService to check if the A11yService warning is required.

This new location provides a single source of truth for whether the
warning must be shown.

A11yManagerService is the central system service with complete knowledge
of the state of enabled services & service shortcuts. Clients (SysUI,
Settings, etc.) should not be calculating this decision themselves.

Note: renames flag to cleanup_accessibility_warning_dialog.
This flag is not yet rolled out so the rename is effectively
just deleting the old flag & adding a new one.

NO_IFTTT=New IFTTT tags

Bug: 303511250
Test: atest AccessibilityShortcutChooserActivityTest
Test: atest AccessibilityManagerServiceTest
Test: m RunSettingsRoboTests ROBOTEST_FILTER=ToggleAccessibilityServicePreferenceFragmentTest
Change-Id: I76b012443cf510b36cc5b3f7f9ed0a7731312a06