Settings: Clean up WifiManager APIs

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

Changed method signatures to conform to API guidelines.

Also removed dead code: WifiManager#getWifiState()
never returns WifiManager#ERROR.
Remove reference to it, since it is an @hide API.

Bug: 143970861
Test: compiles
Change-Id: If4294d106f0acb5c67b5eab1f6f99bc52efc10f2

src/com/android/settings/wifi/RequestToggleWiFiActivity.java

@@ -29,7 +29,6 @@ import android.net.wifi.WifiManager;
 import android.os.Bundle;
 import android.text.TextUtils;
 import android.util.Log;
-import android.widget.Toast;
 
 import androidx.annotation.NonNull;
 
@@ -313,11 +312,6 @@ public class RequestToggleWiFiActivity extends AlertActivity
                         finish();
                     }
                 } break;
-
-                case WifiManager.ERROR: {
-                    Toast.makeText(activity, R.string.wifi_error, Toast.LENGTH_SHORT).show();
-                    finish();
-                } break;
             }
         }
     }