Add AVD creation configs for Android Nougat

Also update `avd create` so that
* Only call device.WaitUntilFullyBooted in `avd start` when
  ensure_system_settings is true otherwise `avd create` will keep timing
  out during instance.Start
* Run the commands for disabling network as root otherwise the command
  will fail with "Killed"

Bug: 1303439
Change-Id: Idab754ef8ac2553f48aceff63528d01c86f0c3cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3508854
Reviewed-by: Peter Wen <wnwen@chromium.org>
Commit-Queue: Haiyang Pan <hypan@google.com>
Cr-Commit-Position: refs/heads/main@{#979044}