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_unittest.cc b/update_attempter_unittest.cc
index 4928477..4ebc85c 100644
--- a/update_attempter_unittest.cc
+++ b/update_attempter_unittest.cc
@@ -31,10 +31,10 @@
 #include <policy/libpolicy.h>
 #include <policy/mock_device_policy.h>
 
-#if USE_LIBCROS
+#if USE_CHROME_NETWORK_PROXY
 #include "network_proxy/dbus-proxies.h"
 #include "network_proxy/dbus-proxy-mocks.h"
-#endif // USE_LIBCROS
+#endif  // USE_CHROME_NETWORK_PROXY
 #include "update_engine/common/fake_clock.h"
 #include "update_engine/common/fake_prefs.h"
 #include "update_engine/common/mock_action.h"
@@ -187,13 +187,13 @@
   brillo::BaseMessageLoop loop_{&base_loop_};
 
   FakeSystemState fake_system_state_;
-#if USE_LIBCROS
+#if USE_CHROME_NETWORK_PROXY
   NetworkProxyServiceInterfaceProxyMock network_proxy_service_proxy_mock_;
   UpdateAttempterUnderTest attempter_{&fake_system_state_,
                                       &network_proxy_service_proxy_mock_};
 #else
   UpdateAttempterUnderTest attempter_{&fake_system_state_, nullptr};
-#endif  // USE_LIBCROS
+#endif  // USE_CHROME_NETWORK_PROXY
   OpenSSLWrapper openssl_wrapper_;
   CertificateChecker certificate_checker_;