Multiple concurrent alerts can not be dismissed

- IMF RTC work item number: 15407
- Apache Cordova Jira issue: https://issues.apache.org/jira/browse/CB-2500
- Markmail or Google Group reference: n/a
- Description: since WP8 alert is nonblocking, multiple alerts can
  be issued concurrently. The second alert gets stuck and you
  can't get out unless you press the back button, which closes the
  application.