Split payload application code into a subdirectory.

This patch splits from the main libupdate_engine code the part that
is strictly used to download and apply a payload into a new static
library, moving the code to subdirectories. The new library is divided
in two subdirectories: common/ and payload_consumer/, and should not
depend on other update_engine files outside those two subdirectories.
The main difference between those two is that the common/ tools are more
generic and not tied to the payload consumer process, but otherwise they
are both compiled together.

There are still dependencies from the new libpayload_consumer library
into the main directory files and DBus generated files. Those will be
addressed in follow up CLs.

Bug: 25197634
Test: FEATURES=test emerge-link update_engine; `mm` on Brillo.

Change-Id: Id8d0204ea573627e6e26ca9ea17b9592ca95bc23
diff --git a/update_attempter.h b/update_attempter.h
index 5debf21..b08b8a8 100644
--- a/update_attempter.h
+++ b/update_attempter.h
@@ -29,13 +29,13 @@
 #include <gtest/gtest_prod.h>  // for FRIEND_TEST
 
 #include "debugd/dbus-proxies.h"
-#include "update_engine/action_processor.h"
 #include "update_engine/chrome_browser_proxy_resolver.h"
 #include "update_engine/client_library/include/update_engine/update_status.h"
-#include "update_engine/download_action.h"
+#include "update_engine/common/action_processor.h"
 #include "update_engine/libcros_proxy.h"
 #include "update_engine/omaha_request_params.h"
 #include "update_engine/omaha_response_handler_action.h"
+#include "update_engine/payload_consumer/download_action.h"
 #include "update_engine/proxy_resolver.h"
 #include "update_engine/system_state.h"
 #include "update_engine/update_manager/policy.h"