NetworkStatsService: disabled by default

For now, there is still too much breakage around the kernel's new
qtaguid netfilter.
So no netstats up front.

But enable on the device until wiped:

  ./vendor/google/tools/override-gservices secure:netstats_enabled=1
  adb reboot


Bug: 4903513

Change-Id: I230bd2d4efab85cac5c9abaa7d638c94f5e0c7d6
Signed-off-by: JP Abgrall <jpa@google.com>