CB-6837 Fix leaked window when hitting back button while alert being rendered

Keep track of the last AlertDialog showed.
The last dialog showed that is rendered while hitting back button it
causes a leaked window.
Instead of perform a full track of all dialogs created, only destroy the
last one showed, this fixes the problem.

close #122

(cherry picked from commit 890e12c30674d7b4dff54082e9ee879505475e19)
Fix leaked window, when hitting back button while alert being rendered

- Cordova RTC work item number: 43292
- Apache Cordova Jira issue:  https://issues.apache.org/jira/browse/CB-6837
- Markmail or Google Group reference: n/a
- Product PMR: n/a
- Worklight RTC work item number: 42684
- Cherry-pick: 890e12c30674d7b4dff54082e9ee879505475e19
- Description: When hitting the back button while alert dialog is being rendered causes a leaked window,
when resuming the app by clicking on its icon using the home screen or using the Multitask button,
the webview doesn't render completely, it shows a blackscreen.
This is get fixed by keeping track of the last AlertDialog showed.
The last dialog showed that is rendered while hitting back button it causes a leaked window,
so during onDestroy event, the webview will only destroy the last one showed, avoiding
the exception and fixing the problem.