Stop and start netd explicitly for "adb shell stop/start".

netd is supposed to be restarted when the zygote is restarted (see the
"onrestart" section for "service zygote" in init.zygote*.rc). But this
only works if you send a restart command (say via "ctl.restart").

"stop && start" != "restart". It seems ingrained in developers to do
"stop && start", so we don't have much hope of convincing everyone to
switch to "adb shell restart", even if we did add such a toolbox command.

Bug: 15855807
Change-Id: I387fe86600f4a2862abc3a05a2ef9a1e7374e21d