[WIFI] Add WiFi subsystem restart callbacks

Add callbacks to indicate the beginning and end of the WiFi subsystem
restart.

ACTS constants for the new callbacks.

Bug: 175084231
Test: N/A - builds.
Change-Id: I14b3fa51d078d90dfc2712a02babd4bc100c7bc7
diff --git a/acts_tests/acts_contrib/test_utils/wifi/wifi_constants.py b/acts_tests/acts_contrib/test_utils/wifi/wifi_constants.py
index 36714f8..517f18d 100644
--- a/acts_tests/acts_contrib/test_utils/wifi/wifi_constants.py
+++ b/acts_tests/acts_contrib/test_utils/wifi/wifi_constants.py
@@ -27,6 +27,8 @@
 WIFI_NETWORK_CB_ON_UNAVAILABLE = "WifiManagerNetworkCallbackOnUnavailable"
 WIFI_NETWORK_CB_ON_LOST = "WifiManagerNetworkCallbackOnLost"
 WIFI_NETWORK_SUGGESTION_POST_CONNECTION = "WifiNetworkSuggestionPostConnection"
+WIFI_SUBSYSTEM_RESTARTING = "WifiSubsystemRestarting"
+WIFI_SUBSYSTEM_RESTARTED = "WifiSubsystemRestarted"
 
 # These constants will be used by the ACTS wifi tests.
 CONNECT_BY_CONFIG_SUCCESS = 'WifiManagerConnectByConfigOnSuccess'