Set variables per variant instead of globally

[timmutton]

Ive begun work on adding the ability to use the proguard files, mapping file and minifyEnabled variables from the variant when a global value isnt supplied, however some things cant be configured on a per-variant basis, such as passes. Ideally, I'd like to move to a model where all variables are configured per variant and get rid of global configuration. In theory, you can set extension variables on gradle objects, but I cant find a way to support it currently. It may require switching to groovy, or using the new android build tools version