SystemUI: Clean up WifiManager APIs

Promoting WifiManager#registerSoftApCallback() and
WifiManager#registerTrafficStateCallback()
to @SystemApi.

Changed method signatures to conform to API guidelines.

Bug: 143970861
Test: atest HotspotControllerImplTest
Change-Id: I1950173be3a62fc1cf8b745f97a503c386a11972
diff --git a/packages/SystemUI/src/com/android/systemui/statusbar/policy/WifiSignalController.java b/packages/SystemUI/src/com/android/systemui/statusbar/policy/WifiSignalController.java
index a441f66..5a43321 100644
--- a/packages/SystemUI/src/com/android/systemui/statusbar/policy/WifiSignalController.java
+++ b/packages/SystemUI/src/com/android/systemui/statusbar/policy/WifiSignalController.java
@@ -128,7 +128,7 @@
     }
 
     public void handleBootCompleted() {
-        mWifiManager.registerTrafficStateCallback(new WifiTrafficStateCallback(), null);
+        mWifiManager.registerTrafficStateCallback(new WifiTrafficStateCallback());
     }
 
     /**