Move NetworkStack start to AM.systemReady

In practice the NetworkStack was not started after
ActivityManager#systemReady, since ActivityManager was only scheduling
the start (mPendingStarts).
Requesting the start too early causes issues for the PackageManager to
install updates/rollbacks.

Test: flashed AOSP and internal builds, WiFi working
Bug: 128331883
Change-Id: I93315ad31925bd436e21d1eabb21d5967e6aae60