Implement policy transparency for CredMan

This CL implements policy transparency for
Credential Manager settings.

Screenshots:
https://hsv.googleplex.com/5502938857340928
https://hsv.googleplex.com/4999933426925568

Test: on device w/ test dpc
Bug: 318552220
Change-Id: I78e1fa47541c81e6e2d2bf2c13159be01928e77c