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_manager/chromeos_policy.cc b/update_manager/chromeos_policy.cc
index 3a81059..6ba6f82 100644
--- a/update_manager/chromeos_policy.cc
+++ b/update_manager/chromeos_policy.cc
@@ -24,11 +24,11 @@
 #include <base/strings/string_util.h>
 #include <base/time/time.h>
 
-#include "update_engine/error_code.h"
+#include "update_engine/common/error_code.h"
+#include "update_engine/common/utils.h"
 #include "update_engine/update_manager/device_policy_provider.h"
 #include "update_engine/update_manager/policy_utils.h"
 #include "update_engine/update_manager/shill_provider.h"
-#include "update_engine/utils.h"
 
 using base::Time;
 using base::TimeDelta;
@@ -533,7 +533,7 @@
 
 EvalStatus ChromeOSPolicy::P2PEnabled(EvaluationContext* ec,
                                       State* state,
-                                      std::string* error,
+                                      string* error,
                                       bool* result) const {
   bool enabled = false;
 
@@ -569,7 +569,7 @@
 
 EvalStatus ChromeOSPolicy::P2PEnabledChanged(EvaluationContext* ec,
                                              State* state,
-                                             std::string* error,
+                                             string* error,
                                              bool* result,
                                              bool prev_result) const {
   EvalStatus status = P2PEnabled(ec, state, error, result);