Allow to Suspend/Resume the ActionProcessor.

This patch implements the core functionality of suspend/resume actions
from the ActionProcessor. No actions support suspend/resume yet.

Bug: 27047026
TEST=Added unittets, tested on edison-eng.

Change-Id: Ib9600098dbccf05fc30f10f0add4a5bc87892b66
diff --git a/update_attempter_unittest.cc b/update_attempter_unittest.cc
index a0977b1..35bd206 100644
--- a/update_attempter_unittest.cc
+++ b/update_attempter_unittest.cc
@@ -38,6 +38,8 @@
 #include "libcros/dbus-proxy-mocks.h"
 #include "update_engine/common/fake_clock.h"
 #include "update_engine/common/fake_prefs.h"
+#include "update_engine/common/mock_action.h"
+#include "update_engine/common/mock_action_processor.h"
 #include "update_engine/common/mock_http_fetcher.h"
 #include "update_engine/common/mock_prefs.h"
 #include "update_engine/common/platform_constants.h"
@@ -45,8 +47,6 @@
 #include "update_engine/common/test_utils.h"
 #include "update_engine/common/utils.h"
 #include "update_engine/fake_system_state.h"
-#include "update_engine/mock_action.h"
-#include "update_engine/mock_action_processor.h"
 #include "update_engine/mock_p2p_manager.h"
 #include "update_engine/mock_payload_state.h"
 #include "update_engine/payload_consumer/filesystem_verifier_action.h"