AAPT2: XML schema and an example configuraiton

The current iteration of the schema allows for both Split and Optimize
commands running off the same file. A split command is multiple optimize
command, and an optimize is a split with only a single artifact. To
combine the two, a single file with one or more artifacts can be passed
to either command. If multiple artifiacts are used for an optimize
command, the required artifact can be specified on the command line. A
configuration with only a single artifact can be used with either
command to get the same result.

Change-Id: Ie48eda6e2f9f8bf25d75ef7bcc3b3401a2d3e8eb
Bugs: 37847908
Test: xmllint --noout --schema aapt2.xsd example/config.xml