wifi(implementation): Remove support for STA + STA in chip combination

Not planning to support STA + STA in Pixel 2018, so remove support for
it from the chip combination.

Also, fixed an error in existing unit tests (Need to mock
WifiLegacyHal.registerRadioModeChangeCallbackHandler)

Bug: 74079118
Test: Tested STA connection & AP bringup on taimen.
Test: ./hardware/interfaces/wifi/1.2/default/tests/runtests.sh
Change-Id: I996e63ecdda4d6a5a07a40d5f7ed1072ada37597
diff --git a/wifi/1.2/default/wifi_legacy_hal.h b/wifi/1.2/default/wifi_legacy_hal.h
index bd68bcd..dedbbf8 100644
--- a/wifi/1.2/default/wifi_legacy_hal.h
+++ b/wifi/1.2/default/wifi_legacy_hal.h
@@ -274,7 +274,7 @@
     wifi_error deregisterErrorAlertCallbackHandler(
         const std::string& iface_name);
     // Radio mode functions.
-    wifi_error registerRadioModeChangeCallbackHandler(
+    virtual wifi_error registerRadioModeChangeCallbackHandler(
         const std::string& iface_name,
         const on_radio_mode_change_callback& on_user_change_callback);
     // RTT functions.