utils.android: force as_root=False when connected_as_root

Some Android devices do not have 'su'. But if they are already
rooted, there's no reason to fail. Circumvent this scenario by never
using 'su' for device already rooted.

devlib/utils/android.py

@@ -333,6 +333,8 @@ class AdbConnection(ConnectionBase):
     # pylint: disable=unused-argument
     def execute(self, command, timeout=None, check_exit_code=False,
                 as_root=False, strip_colors=True, will_succeed=False):
+        if as_root and self.connected_as_root:
+            as_root = False
         try:
             return adb_shell(self.device, command, timeout, check_exit_code,
                              as_root, adb_server=self.adb_server, su_cmd=self.su_cmd)
@@ -351,6 +353,8 @@ class AdbConnection(ConnectionBase):
                 raise
 
     def background(self, command, stdout=subprocess.PIPE, stderr=subprocess.PIPE, as_root=False):
+        if as_root and self.connected_as_root:
+            as_root = False
         bg_cmd = self._background(command, stdout, stderr, as_root)
         self._current_bg_cmds.add(bg_cmd)
         return bg_cmd
@@ -413,6 +417,9 @@ class AdbConnection(ConnectionBase):
         logger.debug("ls command is set to {}".format(self.ls_command))
 
     def _setup_su(self):
+        # Already root, nothing to do
+        if self.connected_as_root:
+            return
         try:
             # Try the new style of invoking `su`
             self.execute('ls', timeout=self.timeout, as_root=True,