a11y: Hide magnification settings during setup wizard

It marks magnification mode and cursor following mode settings
unavailable during setup wizard to be consistent with other
magnification settings.

In addition, this also fixes a linter warning in
`ToggleScreenMagnificationPreferenceFragment`.

Bug: b/401309377
Flag: com.android.settings.accessibility.enable_magnification_cursor_following_dialog
Test: Existing tests
Change-Id: I3c02b2bc50593074a0cb9148c0e42cfde660b980