Fix incorrect doc for WifiInfo#getSSID().

This method will never return null. It may return "<unknown ssid>" if
no network is currently connected.

Change-Id: Iae09a6b2376e47c2d010f8d05750a16361510cca

wifi/java/android/net/wifi/WifiInfo.java

@@ -286,7 +286,7 @@ public class WifiInfo implements Parcelable {
      * Returns the service set identifier (SSID) of the current 802.11 network.
      * If the SSID can be decoded as UTF-8, it will be returned surrounded by double
      * quotation marks. Otherwise, it is returned as a string of hex digits. The
-     * SSID may be {@code null} if there is no network currently connected.
+     * SSID may be <unknown ssid> if there is no network currently connected.
      * @return the SSID
      */
     public String getSSID() {