Add Restricted Mode settings observer to NetworkPolicyManagerService

This allows to enable / disable restricted mode at runtime.

Test: adb shell settings put global restricted_networking_mode 1
Bug: 170322816
Bug: 157505406
Change-Id: Ie9456f6a0b8d6d4e8e4bdfc3f6954e247fd9c5fb
Merged-In: Ie9456f6a0b8d6d4e8e4bdfc3f6954e247fd9c5fb