[NAN] Do not expose WifiManager.isNanSupported as new API

The WifiManager.isXxxSupported() only functions if HAL is enabled
and seems to be a duplicate of:

PackageManager.hasSystemFeature(PackageManager.FEATURE_WIFI_NAN)

Bug: 28251988
Change-Id: I8f410def6f0bd3d3fc0926072ab66acce52a0e14

wifi/java/android/net/wifi/WifiManager.java

@@ -1109,7 +1109,7 @@ public class WifiManager {
 
     /**
      * @return true if this adapter supports Neighbour Awareness Network APIs
-     * @hide PROPOSED_NAN_API
+     * @hide
      */
     public boolean isNanSupported() {
         return isFeatureSupported(WIFI_FEATURE_NAN);