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_manager/real_system_provider.cc b/update_manager/real_system_provider.cc
index 9b968ca..fdf7e86 100644
--- a/update_manager/real_system_provider.cc
+++ b/update_manager/real_system_provider.cc
@@ -20,9 +20,9 @@
 #include <base/callback.h>
 #include <base/logging.h>
 #include <base/time/time.h>
-#if USE_LIBCROS
+#if USE_CHROME_KIOSK_APP
 #include <libcros/dbus-proxies.h>
-#endif
+#endif  // USE_CHROME_KIOSK_APP
 
 #include "update_engine/common/utils.h"
 #include "update_engine/update_manager/generic_variables.h"
@@ -124,7 +124,7 @@
 
 bool RealSystemProvider::GetKioskAppRequiredPlatformVersion(
     string* required_platform_version) {
-#if USE_LIBCROS
+#if USE_CHROME_KIOSK_APP
   brillo::ErrorPtr error;
   if (!libcros_proxy_->GetKioskAppRequiredPlatformVersion(
           required_platform_version, &error)) {
@@ -132,7 +132,7 @@
     required_platform_version->clear();
     return false;
   }
-#endif
+#endif  // USE_CHROME_KIOSK_APP
 
   return true;
 }