Fix NPE in SecurityPolicy.policiesRequired

* Add check for null Account, as this method can be called from a
  background thread, and the Account might have been deleted by the
  time we're called

Bug: 3396365
Change-Id: Ie125ed714c73d51beaedc818b6b731cea941666f

src/com/android/email/SecurityPolicy.java

@@ -447,6 +447,8 @@ public class SecurityPolicy {
      */
     public void policiesRequired(long accountId) {
         Account account = EmailContent.Account.restoreAccountWithId(mContext, accountId);
+        // In case the account has been deleted, just return
+        if (account == null) return;
 
         // Mark the account as "on hold".
         setAccountHoldFlag(mContext, account, true);