Update Android getting started instructions for new folder layout

There is no longer an xwalk_app_template.tar.gz file to unpack
after downloading Crosswalk Android (the top-level directory
after you've unpacked is the app template).

Fix the instructions, removing all references to this second
unpacking stage.
Merge pull request #96 from townxelliot/new-dl-structure

Update Android getting started instructions for new folder layout
Add note to FAQ about support for desktop platforms
Update build instructions

This addresses https://crosswalk-project.org/jira/browse/XWALK-1307
and https://crosswalk-project.org/jira/browse/XWALK-1059.