[native] Pull useIsCurrentUserStaff() out of useStaffCanSee()

Summary: Pull useIsCurrentUserStaff() out from useStaffCanSee() and consume in BuildInfo and StaffContextProvider.

Test Plan: BuildInfo + StaffContextProvider continue to look/work as expected.

Reviewers: tomek, marcin, varun, ginsu, rohan, O2 Blocking Reviewers

Reviewed By: tomek, O2 Blocking Reviewers

Subscribers: ashoat, abosh

Differential Revision: https://phab.comm.dev/D5587