Fix javadoc error for requestNetwork.

The ConnectivityManager documentation uses {@link requestNetwork}
without specifying which version of that method it refers to.
This results in javadoc incorrectly linking to the version of the
method that takes a PendingIntent instead of the version that
takes a callback.

Change-Id: Ia914ec88005a6401b6391c5b4fc92f988baa3922