Support cross-user VPN calls (with permission)

Settings and SystemUI need to act on other users than USER_OWNER.

This is gated by INTERACT_ACROSS_USERS_FULL in addition to the existing
CONTROL_VPN checks, so the number of processes able to interfere with
other profiles' VPNs should be quite small.

Bug: 20692490
Bug: 20747154
Bug: 20872408
Change-Id: I6e5d7220f73435bec350719e7b4715935caf4e19