Eclipse plugin can reuse the Android SDK location.

Add getSdkPath() method in ExportHelper.java. Get SDK Path from ADT preference.
Instead of input sdk path manually.

BUG=XWALK-1929
Sorry, but the English here still needs some improvement :)
How about _Crosswalk's path has not been set correctly. Please go to Window->Preferences->Crosswalk App to configure it_.

Ping @townxelliot for some native proof-reading?