Add mutability flag to WifiSlice.

Starting Android S, PendeingIntent should have a explicit mutability
flag to avoid vulnerability.

Fixes: 172204181
Test: 1. adb shell am start -a
android.settings.panel.action.INTERNET_CONNECTIVITY and play on the wifi
panel.
2. Enable contextualWifiSlice and play on it.
3. Adjust volume on the volume panel.

Change-Id: I23f03cae8c481d9f5836b7fb967f734e9c99cd65

src/com/android/settings/wifi/slice/WifiSlice.java

@@ -232,7 +232,7 @@ public class WifiSlice implements CustomSliceable {
             CharSequence title) {
         intent.addFlags(Intent.FLAG_RECEIVER_FOREGROUND);
         final PendingIntent pi = PendingIntent.getBroadcast(mContext, requestCode, intent,
-                PendingIntent.FLAG_UPDATE_CURRENT);
+                PendingIntent.FLAG_UPDATE_CURRENT | PendingIntent.FLAG_IMMUTABLE);
         return SliceAction.create(pi, icon, ListBuilder.ICON_IMAGE, title);
     }