Add Restricted Mode uid allowlist setting observer to NetworkPolicyManagerService

This allows to add / remove UIDs from restricted mode's allowlist at runtime.

Bug: https://issuetracker.google.com/issues/206947902
Test: adb shell settings put global uids_allowed_on_restricted_networks "UID"
Change-Id: I1d3edf3fb1ee62ddf9ee07e15cf1da1e23cc18fa
(cherry picked from commit 54217bfab16a9201f35bb1e9be8b26bb56a5809e)