Replace MAX_NETWORK_TYPE with TYPE_VPN

ConnectivityManager#MAX_NETWORK_TYPE is a hidden API which cannot
be accessed after ConnectivityManager becomes a part of mainline
module.
Use the max public network type - TYPE_VPN should be enough since
TYPE_* is deprecated and no longer extended.

Bug: 182859030
Test: 1. Remove "framework-connectivity.impl"
      2. cd frameworks/base/core/tests/bandwidthtests; mm
Change-Id: I27579a942a0424db647ad1f3383db78d5a0c69a9