Fix kernel net tests fail in user build

Before this change, system routing only can be bypassed by
the debug build process. This change let the process running
as root can bypass system routing if this process have
specific env flag. In other word, the other processes own by
root would not be affected if they don't set the specific env
flag.

Bug: 135422468
Test: run vts -m VtsKernelNetTest in both user and eng build
Change-Id: I39d0b0141ef51c6f16052ffc785d1d2f523cf11f