Fix issue #62342672: API Review: OPSTR_ACTIVATE_VPN SystemApi

Add a new platform-only permission for being able to change
app ops mode, so nothing outside of the platform can do this.

Bug: 62342672
Test: Booted, ran, settings works, shell works, apps install
Change-Id: I53286a6102d50d0b8f286498b4d2e6eea416c0ae