Delay test for APF activation after screen-off

CTS tests screen-on device in the beginning, but APF is not activated after screen-off in the test setup.
This change adds 3 secs delay to ensure APF activation before test execution.

Bug: 402667362
Test: ./cts-tradefed run cts-on-gsi -m CtsConnectivityMultiDevicesTestCases -t ApfV4Test --shard-count 2
Test: atest CtsConnectivityMultiDevicesTestCases:ApfV4Test
Change-Id: I067ce792b5f89e2f0051933982ebc734607fbf98
diff --git a/staticlibs/testutils/host/python/apf_test_base.py b/staticlibs/testutils/host/python/apf_test_base.py
index 33b3838..6a62e21 100644
--- a/staticlibs/testutils/host/python/apf_test_base.py
+++ b/staticlibs/testutils/host/python/apf_test_base.py
@@ -70,6 +70,9 @@
     # Enable doze mode to activate APF.
     adb_utils.set_doze_mode(self.clientDevice, True)
 
+    # wait for APF to become active.
+    time.sleep(3)
+
   def teardown_class(self):
     adb_utils.set_doze_mode(self.clientDevice, False)
     tether_utils.cleanup_tethering_for_upstream_type(