[Android] Open web pages in default browser instead of in-app browsing.

Pages linked with <a target="_blank"> or windowd.open() will be opened
in system default browser.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1037
Merge pull request #2168 from gaochun/1037

[Android] Open web pages in default browser instead of in-app browsing.