Deprecate WebView authentication flow

[lbalmaceda]

Methods that receive a REQUEST_CODE are now deprecated. Users should avoid calling builder.useBrowser(false).

//Start the authentication
public void doAuth(){
   WebAuthProvider.init(account)
      .start(activity, callback);
}

//Capture the result and resume the authentication
@Override
protected void onNewIntent(Intent intent) {
   if (WebAuthProvider.resume(intent)) {
      return;   //Handled
   }
   super.onNewIntent(intent);
}