Add a constant for system_time to DeviceConfig

Add a constant for "system_time" to DeviceConfig for server-push feature
flags. "system_time" has been added server-side under cl/353067789.

This change references the new namespace for device-side config code.

This change is following the go/android-flags-setup process. Flags will
be added in follow up changes.

Bug: 159121373
Bug: 178107773
Test: build only
Change-Id: If49365cddc5827382ea4c466322c9fc65777d564