update_engine: Breaks the use of libcros into two dbus proxies.

This CL breaks USE_LIBCROS into two defines:
	USE_CHROME_NETWORK_PROXY
	USE_CHROME_KIOSK_APP
and changes all related build artifacts for both chrome and android.

Breaks USE_libcros into USE_chrome_network_proxy and USE_chrome_kiosk_app.
Removes BRILL_USE_LIBCROS.
Replaces __BRILLO__ with USE_OMAHA.

BUG=chromium:717306
TEST=Ran test 'cros_workon_make --board=amd64-generic --test update_engine'
	for all four conditions of the newly introduced two flags.

Change-Id: I9ca5b35c22a17c45a861db6a434239096a896127
Reviewed-on: https://chromium-review.googlesource.com/596802
Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Tested-by: Sen Jiang <senj@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>
diff --git a/update_attempter.cc b/update_attempter.cc
index ff3b046..9cf9368 100644
--- a/update_attempter.cc
+++ b/update_attempter.cc
@@ -126,12 +126,12 @@
         network_proxy_service_proxy)
     : processor_(new ActionProcessor()),
       system_state_(system_state),
-#if USE_LIBCROS
+#if USE_CHROME_NETWORK_PROXY
       cert_checker_(cert_checker),
       chrome_proxy_resolver_(network_proxy_service_proxy) {
 #else
       cert_checker_(cert_checker) {
-#endif  // USE_LIBCROS
+#endif  // USE_CHROME_NETWORK_PROXY
 }
 
 UpdateAttempter::~UpdateAttempter() {