Fixes contrast on A11yMenu settings switch by using Settings theme.

This switch now matches the same color scheme as used by the core
Settings app, in both light and dark mode.

Also, as allowed by "Flag Cleanup Policy", this change finishes
the rollout of nextfood flag
a11y_menu_settings_back_button_fix_and_large_button_sizing,
by defaulting to the "true" state and deleting from aconfig.

Fix: 319761382
Test: Inspect using contrast picker
Flag: None (low risk UI contrast fix)
Change-Id: Ib084f4a9d55365bf0a7269f337ba1f568c2cad0c