Persist pingHeartbeat after pingLoop finishes

* We weren't persisting the current heartbeat after the loop finishes,
  so we'd end up starting next time with the default start heartbeat
* The new code is a bit more efficient, as we'd start the next pingLoop
  with the heartbeat previously in effect

Bug: 2492854
Change-Id: I1d488e3eb05530c452605b25108b35af582e692f

src/com/android/exchange/EasSyncService.java

@@ -1507,6 +1507,9 @@ public class EasSyncService extends AbstractSyncService {
                 sleep(30*MINUTES, true);
             }
         }
+
+        // Save away the current heartbeat
+        mPingHeartbeat = pingHeartbeat;
     }
 
     void sleep(long ms, boolean runAsleep) {