Better check for if device has root

"which su" will return the path of the su binary, or it will return nothing. 
The python boolean of a string with something in it (such as the path of the su binary), will be True.
An empty string (where there is no su binary) will be False.

mvt/android/modules/adb/base.py

@@ -96,7 +96,7 @@ class AndroidExtraction(MVTModule):
         """Check if we have a `su` binary on the Android device.
         :returns: Boolean indicating whether a `su` binary is present or not
         """
-        return bool(self._adb_command("[ ! -f /sbin/su ] || echo 1"))
+        return bool(self._adb_command("which su"))
 
     def _adb_root_or_die(self):
         """Check if we have a `su` binary, otherwise raise an Exception.