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: I372e649c019a8f9b95919ff0da6f56612d7061c2