Add D-Bus method to get the duration since an update completed.

This new API can be used by Chrome to automatically reboot the device
at N hours after updating. This is implemented as a D-Bus method that
returns the number of micro-seconds on the wall-clock since the update
completed. If the device has not updated, the D-Bus method returns an
error.

For robustness, durations are measured using the CLOCK_BOOTTIME clock
instead of the usual CLOCK_REALTIME clock. This avoids interference
with NTP adjustments, the RTC clock being wrong and other things.

BUG=chromium:218192
TEST=New unit test + unit tests pass + manual test on a device using
    the gdbus(1) command as the chronos user.

Change-Id: I51d44d69afe2d3024bb0780916c3c4e3f8ebb19e
Reviewed-on: https://chromium-review.googlesource.com/173032
Reviewed-by: David Zeuthen <zeuthen@chromium.org>
Commit-Queue: David Zeuthen <zeuthen@chromium.org>
Tested-by: David Zeuthen <zeuthen@chromium.org>
diff --git a/UpdateEngine.conf b/UpdateEngine.conf
index dee64ee..3688250 100644
--- a/UpdateEngine.conf
+++ b/UpdateEngine.conf
@@ -49,6 +49,9 @@
     <allow send_destination="org.chromium.UpdateEngine"
            send_interface="org.chromium.UpdateEngineInterface"
            send_member="GetUpdateOverCellularPermission"/>
+    <allow send_destination="org.chromium.UpdateEngine"
+           send_interface="org.chromium.UpdateEngineInterface"
+           send_member="GetDurationSinceUpdate"/>
     <allow send_interface="org.chromium.UpdateEngineLibcrosProxyResolvedInterface" />
   </policy>
   <policy context="default">