Fix testApfFilterKeepaliveAck.

Apf program for TCP keep alive ACK also compares protocol field in IPv4
header and the total length of TCP keep alive ACK packet. Besides,
should set the ACK flag in the the TCP header of keep alive ACK packet.

Bug: 123992564
Test: atest NetworkStackTests
Change-Id: I997cd6807a33f7e1af25566226d803dcc2e257dd