Don't notify new users if there is a pending SQLCipher migration.

If a user is upgrading to use the SQLCipher database (which happened
back in 4.16, so this only applies to relatively dormant users who are
just getting back into the app) and received a new "user X joind signal"
message, then it could screw up the migration. So we're just dropping
these notifications that happen in this narrow window.

src/org/thoughtcrime/securesms/util/DirectoryHelper.java

@@ -142,7 +142,7 @@ public class DirectoryHelper {
         ApplicationContext.getInstance(context).getJobManager().add(new MultiDeviceContactUpdateJob(context));
       }
 
-      if (!activeUser && systemContact) {
+      if (!activeUser && systemContact && !TextSecurePreferences.getNeedsSqlCipherMigration(context)) {
         notifyNewUsers(context, Collections.singletonList(recipient.getAddress()));
       }