VPN network stat accounting changes.

Properly account for VPN apps that make heavy use of the tun
interface. Prior to this change a VPN app could be incorrectly charged
for more data than it actually used if it sent more traffic through
the tun interface than the underlying interface.

This change excludes VPN app traffic on the tun interface from the
adjustment pool and doesn't redistribute traffic to the VPN app.
Instead all of the redistributed traffic is deducted from the VPN app
which effectively represents any overhead incurred by the VPN app.

BUG: 30557871
Change-Id: I62a75a0c0c0111e052b7903baa9f5d6d94ef57fd