[Android] Fix that window.close is not working

Webapp might use window.close() to exit app.
This commit turns on the setting to allow this
usage and finish the activity when calling
window.close().

BUG=https://crosswalk-project.org/jira/browse/XWALK-939
Merge pull request #1578 from wang16/window_close

[Android] Fix that window.close is not working