Change the icon of connected notification

Updated the icon_wifi.xml to follow the old design which was
referring to the ic_wifi_signal_4.xml.

Bug: 150903640
Test: 1. Build pass.
      2. Flash the ROM to see if the icon has changed.

Change-Id: I276f48550631ac654cc7a6426329414ff0fa0963
Merged-In: I276f48550631ac654cc7a6426329414ff0fa0963