Fix build warning due to WebSettings now being abstract

WebSettings was made abstract in
https://android-git.corp.google.com/g/#/c/177360

current.txt was updated, but this causes a build warning because 16.txt was not
updated to match.

Change-Id: I4e00297361048509fab236f292c07d8a0df28f28

api/16.txt

@@ -24979,7 +24979,7 @@ package android.webkit {
     method public void setMimeType(java.lang.String);
   }
 
-  public class WebSettings {
+  public abstract class WebSettings {
     method public boolean enableSmoothTransition();
     method public boolean getAllowContentAccess();
     method public boolean getAllowFileAccess();