Expose AppOpsManager#OPSTR_ACTIVATE_PLATFORM_VPN as system API

Connectivity service module is using some AppOpsManager @hide
APIs but they are not able to call after CS become a
mainline module. Thus, expose them to System APIs.

Bug: 170593746
Test: make update-api
Change-Id: Idb8ee13394ccbd266061eef0cab08a5e28948a3e