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