[WifiChannelSwitchStressTest] Use wifi_test_utils to set country code

Use wifi_test_utils.set_wifi_country_code() to set country code,
instead of WifiManager.sestCountryCode().

Bug: None.
Test: Verified test.
Change-Id: I7f69fcf7263a1018344c189ad4c582a3cb733741
diff --git a/acts/tests/google/wifi/WifiChannelSwitchStressTest.py b/acts/tests/google/wifi/WifiChannelSwitchStressTest.py
index 4e06ca1..f76331e 100644
--- a/acts/tests/google/wifi/WifiChannelSwitchStressTest.py
+++ b/acts/tests/google/wifi/WifiChannelSwitchStressTest.py
@@ -56,7 +56,7 @@
         for ad in self.android_devices:
             wutils.wifi_test_device_init(ad)
             utils.sync_device_time(ad)
-            ad.droid.wifiSetCountryCode(wutils.WifiEnums.CountryCode.US)
+            wutils.set_wifi_country_code(ad, WifiEnums.CountryCode.US)
 
     def setup_test(self):
         for ad in self.android_devices:
@@ -305,4 +305,4 @@
                 "hotspot frequency != client frequency")
             wutils.stop_wifi_tethering(self.dut)
             if hasattr(self, 'packet_capture'):
-                wutils.stop_pcap(self.packet_capture, self.pcap_procs, False)
\ No newline at end of file
+                wutils.stop_pcap(self.packet_capture, self.pcap_procs, False)