Bug 1374758 - Add 'when' epoch timestamp to each sync ping r=nalexander

Since we bundle multiple pings together for an upload, it really helps to know when
each individual ping happened.

MozReview-Commit-ID: 2NuHFiNSHFA

--HG--
extra : rebase_source : 8600483688a90dc79684a15d837e6181a665ed36

mobile/android/base/java/org/mozilla/gecko/telemetry/pingbuilders/TelemetrySyncPingBuilder.java

@@ -141,6 +141,7 @@ public class TelemetrySyncPingBuilder extends TelemetryLocalPingBuilder {
 
     @Override
     public TelemetryLocalPing build() {
+        payload.put("when", System.currentTimeMillis());
         return new TelemetryLocalPing(payload, docID);
     }