Allow libcore to perform necessary actions upon net config changes.

Delegate to libcore the responsibility of performing the necessary
actions upon a network configuration change, such as a DNS
configuration change.  In the L-release, these responsibilities
now include flushing the HTTP connection pools to prevent connections
on lingering networks getting pulled from the pool inadvertently to
satisfy requests that should be going over the new network.  All such
responsibilities can now be controlled and delegated by libcore's
new NetworkEventDispatcher, designed for this purpose.

bug:17314604
Change-Id: Ic796aea9d9a1f6ecb787fbdb38b3b7f5d991829b