devtools/madb: specify devices with their indices.

This allows users to specify devices with their devices indicies. For
example,

    madb -n=@1,@2 start

will only start the app on the first and second devices, and not on
the res.

Change-Id: I96573e50e5693e5ca5abbb7ad4b4d851ee33fa7b
3 files changed