ANDROID: 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: I10545a308d61a2124077cb6f05fb0f5d9b4559ad