Migrate away from AsyncChannel in NetworkAgent

Use two oneway binder interfaces instead.
The interfaces post messages to handlers as was implemented before, but
provide a more strictly defined interface, with less hops between
NetworkAgent, AsyncChannel, and ConnectivityService.

The actual public interface is the NetworkAgent @SystemApi: the binder
interface is an internal implementation detail.

Test: atest FrameworksNetTests CtsNetTestCasesLatestSdk
Change-Id: Ie364ab50f416e7821e70f4539a881eea828e1256