Specify a user for the art_boot service

init complains:
 E init    : /apex/com.android.art/etc/init.rc: 26: No user specified for service 'art_boot'. Defaults to root.

Change Icb0ff95e341a23cf452cfb803fb43f5f46ef316e should not have removed
the 'user' directive. No functional change, but this silences the error.

Test: Boot AOSP and check logcat for the error
Change-Id: I3b44bd5454d878f16509782514c106ff9fd9ceaa