Use command -v instead of which to check for root

`command` is built in, unlike `which`, and is more reliable.
https://github.com/mvt-project/mvt/pull/19#issuecomment-885650430
https://stackoverflow.com/questions/592620/how-can-i-check-if-a-program-exists-from-a-bash-script/677212#677212

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("which su"))
+        return bool(self._adb_command("command -v su"))
 
     def _adb_root_or_die(self):
         """Check if we have a `su` binary, otherwise raise an Exception.