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_request_action_unittest.cc b/omaha_request_action_unittest.cc
index 9bdc167..4897b7a 100644
--- a/omaha_request_action_unittest.cc
+++ b/omaha_request_action_unittest.cc
@@ -11,7 +11,9 @@
 #include <base/stringprintf.h>
 #include "base/time.h"
 #include "gtest/gtest.h"
+
 #include "update_engine/action_pipe.h"
+#include "update_engine/constants.h"
 #include "update_engine/mock_http_fetcher.h"
 #include "update_engine/omaha_hash_calculator.h"
 #include "update_engine/omaha_request_action.h"