[Thread] use dedicated flag for Thread settings

a larger THREAD_ENABLED_PLATFORM flag which guards all Thread-related
platform changes is used for the Thread settings change. But we need
more flexible control of the UX release timeline. So use a dedicated
THREAD_SETTINGS_ENABLED flag here.

Bug: 329384658
Change-Id: Ib5b3ae3c71ccc890fc46af982b021858774938f3