omaha: distinguish zero and unset in ping active days field

Unlike the other fields unset here would mean unknown rather than "0" so
we must distinguish between the two. In the end it isn't very
significant since our update server and none of our clients use these
self-reported active times, exclusively using when pings were received.

omaha/protocol.go

@@ -108,9 +108,9 @@ type UpdateRequest struct {
 }
 
 type PingRequest struct {
-	Active               int `xml:"active,attr,omitempty"`
-	LastActiveReportDays int `xml:"a,attr,omitempty"`
-	LastReportDays       int `xml:"r,attr,omitempty"`
+	Active               int  `xml:"active,attr,omitempty"`
+	LastActiveReportDays *int `xml:"a,attr,omitempty"`
+	LastReportDays       int  `xml:"r,attr,omitempty"`
 }
 
 type EventRequest struct {