[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
The sample was not found in github and the link you provided was not found.  I put the tar.gz in the documentation/samples/ directory.  I suppose this is fine for a tgz.  If you want to make it part of the "Samples" documentation then that would be nice too, but would require an article stub that at least pointed them to here.