Rename LegacyRoleHoldersResolver to LegacyRoleHolderProvider for system API.

LegacyRoleHolderProvider will need to live in the system server to
allow OEMs to provide their custom logic, meanwhile its various usages
of hidden APIs are also unfit for mainline.

Bug: 158736025
Test: presubmit
Change-Id: I979814a8de91b07176e2b5a6e5d4be914cd3c99d