[Android] The app UI is locked if users cancel the JS alert dialog.

This is because the onClickListener of the cancel button is not handled.
As a result, the UI is blocked. Besides, onCancelListener can handle
back key so we don't have to override it.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1627
Merge pull request #1928 from yongsheng/fix1627

[Android] The app UI is locked if users cancel the JS alert dialog.