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/payload_state_unittest.cc b/payload_state_unittest.cc
index e647a8c..9f7f5c1 100644
--- a/payload_state_unittest.cc
+++ b/payload_state_unittest.cc
@@ -8,6 +8,7 @@
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 
+#include "update_engine/constants.h"
 #include "update_engine/omaha_request_action.h"
 #include "update_engine/payload_state.h"
 #include "update_engine/prefs_mock.h"