Refactoring: Move prefs constants out to the constants.h/cc

Adding/removing the set of keys used by various parts of the Update Engine code
does not affect the implementation of the prefs module, hence separating them.

BUG=chromium:229090
TEST=Unit tests pass.

Change-Id: Id774b726a47b461e563e2e413527054e4da0c012
Reviewed-on: https://gerrit.chromium.org/gerrit/47593
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Tested-by: Jay Srinivasan <jaysri@chromium.org>
Commit-Queue: Jay Srinivasan <jaysri@chromium.org>
diff --git a/omaha_response_handler_action_unittest.cc b/omaha_response_handler_action_unittest.cc
index 9f26bca..80dfc5c 100644
--- a/omaha_response_handler_action_unittest.cc
+++ b/omaha_response_handler_action_unittest.cc
@@ -6,6 +6,7 @@
 
 #include <gtest/gtest.h>
 
+#include "update_engine/constants.h"
 #include "update_engine/omaha_response_handler_action.h"
 #include "update_engine/mock_system_state.h"
 #include "update_engine/test_utils.h"