update_engine: add staging pref to payload_state

Add kPrefsWallClockStagingWaitPeriod as a new pref so that the staging
wait period persists accross reboots. Change the old
kPrefsWallClockWaitPeriod constant to
kPrefsWallClockScatteringWaitPeriod to differentiate more clearly
between the two.

BUG=chromium:858621
TEST=cros_workon_make update_engine --test

Change-Id: Ifc6aeb44271321cdd2f0d4cfa583a7bbc32b9004
Reviewed-on: https://chromium-review.googlesource.com/1138741
Commit-Ready: Adolfo Higueros <adokar@google.com>
Tested-by: Adolfo Higueros <adokar@google.com>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: May Lippert <maybelle@chromium.org>
diff --git a/common/constants.h b/common/constants.h
index 78353d8..d97c5fb 100644
--- a/common/constants.h
+++ b/common/constants.h
@@ -90,7 +90,8 @@
 extern const char kPrefsUpdateStateSignedSHA256Context[];
 extern const char kPrefsUpdateTimestampStart[];
 extern const char kPrefsUrlSwitchCount[];
-extern const char kPrefsWallClockWaitPeriod[];
+extern const char kPrefsWallClockScatteringWaitPeriod[];
+extern const char kPrefsWallClockStagingWaitPeriod[];
 
 // Keys used when storing and loading payload properties.
 extern const char kPayloadPropertyFileSize[];