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
diff --git a/Android.bp b/Android.bp
index 62ac6e7..d26c49c 100644
--- a/Android.bp
+++ b/Android.bp
@@ -330,6 +330,7 @@
     srcs: [
         // Java/AIDL sources under frameworks/base
         ":framework-blobstore-sources",
+        ":framework-connectivity-sources", // framework-connectivity is not yet a module
         ":framework-core-sources",
         ":framework-drm-sources",
         ":framework-graphics-nonupdatable-sources",