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/delta_performer.cc b/delta_performer.cc
index 12f0e20..a815e28 100644
--- a/delta_performer.cc
+++ b/delta_performer.cc
@@ -18,6 +18,7 @@
 #include <google/protobuf/repeated_field.h>
 
 #include "update_engine/bzip_extent_writer.h"
+#include "update_engine/constants.h"
 #include "update_engine/delta_diff_generator.h"
 #include "update_engine/extent_ranges.h"
 #include "update_engine/extent_writer.h"