[lib] Introduce decodeRolePermissionBitmask

Summary:
To reverse rolePermissionToBitmaskHex introduced in D9662.

Depends on D9662

Test Plan: Unit tests, will add a few more.

Reviewers: ashoat, ginsu, tomek, rohan

Reviewed By: ashoat

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9663