[Android] Add documentation of Crosswalk for Android AAR

This documentation introduce about using crosswalk aar in Gradle and maven project. Add samples about using AAR with Gladle and Maven Project.

BUG=XWALK-2545
Would be good to have a link.  Also there is an Ubuntu package called maven2 (and also a gradle pkg).  Is that useful to mention or do you want them to use the Android SDK gradle and the upstream maven?