[netstack] Avoid configuring ethernet clients after they've been started

...fixing the race condition causing Chromium test crashes outlined in
NET-2026.
This logic will be moved into netcfg in the work for NET-1837.

Tested:
  - fx run-test netstack_test
  - fx run-test netstack_integration_test
  - manual test on Toulouse: primary NIC gets a DHCP address from
  edgerouter on boot

Bug: NET-2026 #done
Change-Id: I362f2415abc75c781d48fab0adf035fb3240aa52
3 files changed