Refactor getting allowlist set from feature provider.

(1) Use Set<CharSequence> for all the allowlists.
(2) The contains() method of Set<CharSequence> also works for String. No
    need an extra contains() util function.

Bug: 262802531
Fix: 262802531
Test: presubmit
Change-Id: Ib2aaf9a3b8db4618a8c46f138e8d35f15b77c104