Add webservd client for testing purposes

Right now, this client only answers ping requests.  We'll enable more
interesting functionality later.  With this patch and a corresponding
selinux policy modification, we can:

$ adb forward tcp:8080 tcp:80
$ adb shell start webservd_tclient
$ curl http://localhost:8080/webservd-test-client/ping
  Still alive, still alive.

Refactor a little of the build logic to make it easier later on to do
DBus/Binder specific implementations.

Bug: 26385135
Test: compiles, and above shell commands work on emulator

Change-Id: I225793a455972d99e97804d07f9ab7d15ca2b716
6 files changed