source Android jars from a URL

This will require Jenkins changes to be made to set the new ANDROID_URL
environment variable. The Jenkins job that needs changing is here:

http://factory.couchbase.com/view/build/view/mobile_dev/job/package_phonegap_plugin/

The ANDROID_REL and ANDROID_DIR variables should be removed from Jenkins
also.

Change-Id: Ib3519685e8a59756b8a02a6d3be737cb169836b6
Reviewed-on: http://review.couchbase.org/36743
Reviewed-by: Traun Leyden <tleyden@couchbase.com>
Tested-by: Traun Leyden <tleyden@couchbase.com>