[lib] Update roleIsDefaultRole to consider roleInfo?.specialRole

Summary:
Similar to D11148, but for specialRoles.DEFAULT_ROLE.

Depends on D11148

Test Plan:

1. Hard code includeSpecialRoleFieldInRoles to true
2. Check to see that specialRole field of roles are populated in ThreadStore
3. Set a breakpoint and see that the branch is hit and function behaves as expected.

Reviewers: ginsu, tomek, michal, kamil

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11149