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/payload_state_interface.h b/payload_state_interface.h
index 3adc148..d384a0e 100644
--- a/payload_state_interface.h
+++ b/payload_state_interface.h
@@ -205,6 +205,9 @@
 
   // Switch to next payload.
   virtual bool NextPayload() = 0;
+
+  // Sets and persists the staging wallclock-based wait period.
+  virtual void SetStagingWaitPeriod(base::TimeDelta wait_period) = 0;
 };
 
 }  // namespace chromeos_update_engine