Format "su -c" args for the device-specific shell.

SuperSu wants a single arg, e.g. `adb shell su -c 'ls /'`
userdebug wants separate args, e.g. `adb shell su -c ls /`
Fixed 'su' format to support userdebug.
Merge pull request #112 from wrightt/topic/adb_su

Format "su -c" args for the device-specific shell.