Accessibility shortcut improvement (8/n)

- Migrate magnification navibar enabled setting to a11y button targets
  list setting.
- Clear settings of a11y button targets list, since there's no need to
  track legacy ones.

Bug: 136293963
Test: atest SettingsProviderTest
Change-Id: I0e83ebe55e00b5c760e3df5180a5d3a67c429e6f