Fix concurrent modification exception

* Synchronize recently added call to releaseMailbox on sSyncLock

Bug: 2639210
Change-Id: Iaa1f2ed7cdc9cc0c306363487095ac5ccd02f9a3

src/com/android/exchange/SyncManager.java

@@ -1454,7 +1454,9 @@ public class SyncManager extends Service implements Runnable {
                                // In this case, we release the mailbox so that we can start another
                                // thread to do the work
                                log("Alarm failed; releasing mailbox");
+                               synchronized(sSyncLock) {
                                    syncManager.releaseMailbox(id);
+                               }
                                // Shutdown the connection manager; this should close all of our
                                // sockets and generate IOExceptions all around.
                                syncManager.shutdownConnectionManager();