Update OWNERS file for update_engine

ahassani@ left google, remove him from the list

Adding myself as I've been working on update_engine repo for quite a
while

Test: th
Change-Id: Ic22ff922d5e9e72ba337211fd682a5897c7d068d
diff --git a/OWNERS b/OWNERS
index 938752f..58ecfe1 100644
--- a/OWNERS
+++ b/OWNERS
@@ -5,10 +5,11 @@
 elsk@google.com
 senj@google.com
 xunchang@google.com
+zhangkelvin@google.com
 
 # Chromium OS maintainers:
-ahassani@google.com
 kimjae@google.com
 
+
 # Chromium OS only:
 # COMPONENT: Internals>Installer