Use a single interface for reading pre-Q and pre-S role state.

The logic to read the platform XML is moved from RoleUserState. The
logic to read legacy settings is left untouched except refactored to
return the mapping at once.

Bug: 158736025
Test: presubmit
Change-Id: Ifd1770bf869ea77b2f8a4ff8b3eae46a46cc90cf