update android-sdk,add build-tools to var 

I have noticed the last sentence in this formula

      # The 'android' tool insists on deleting #{prefix}/platform-tools
      # and then installing the new one. So it is impossible for us to redirect
      # the SDK location to var so that the platform-tools don't have to be
      # freshly installed EVERY DANG time the base SDK updates.

I think the previous contributors were aware of this problem, so I am not very sure if my PR is CORRECT(or the best way).
But I have verified on my MAC, it works.
I hope someone can evaluate it