Split network agent created state from connected state

Network creation setup sometimes involves extra steps after actually
calling into netd to create the underlying network, rules & routes &
to begin allowing sockets bound to it.

For example, VPN networks can set a UID whitelist or blacklist. This
should happen:

  - AFTER there is a netID & network created in netd as
    network-specific rules will need to be tied to / point at it. Those
    rules are tied to the lifecycle of netd's network which is tracked
    by `NetworkAgentInfo.created` on the frameworks side.

  - BEFORE the CONNECTED broadcast and network callbacks have been sent
    out so that we don't create a race condition between clients that
    want to use the network and the server actually having the network
    ready

The race condition existed prior to this change and required any client
making use of network callbacks to sleep for a short amount of time after
receiving to actually be able to use the network.

Among other things, that race condition is now fixed.

Bug: 26694104
Change-Id: Ied92f5588a98c3e97f456bc98b676bf201ab5472