Refactor ChromeOSPolicy into policy and utilities

Building on the policy fragments that have been previously extracted
from ChromeOSPolicy, this extracts a few more pieces of logic and
replaces sections of ChromeOSPolicy with calls to the extracted
methods.

Bug: 66016687
Test: unit tests, manually triggered OTA updates

Change-Id: I3bc608065f8ab89982f71b8490ebd66ed2266aa3