ANDROID: netfilter: xt_qtaguid: Use sk_uid to replace uid get from socket file

Retrieve socket uid from the sk_uid field added to struct sk instead of
read it from sk->socket->file. It prevent the packet been dropped when
the socket file doesn't exist.

Bug: 37524657
Signed-off-by: Chenbo Feng <fengc@google.com>
Change-Id: Ic58239c1f9aa7e0eb1d4d1c09d40b845fd4e8e57