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/prefs_mock.h b/prefs_mock.h
index 963b981..4f991fc 100644
--- a/prefs_mock.h
+++ b/prefs_mock.h
@@ -6,6 +6,7 @@
 #define CHROMEOS_PLATFORM_UPDATE_ENGINE_PREFS_MOCK_H__
 
 #include "gmock/gmock.h"
+#include "update_engine/constants.h"
 #include "update_engine/prefs_interface.h"
 
 namespace chromeos_update_engine {