Specify SPIs in host byte order in xfrm code.

The result is much easier to read as it removes lots of htonl()
calls.  It's also more intuitive, because the SPIs seen in the
code match those displayed by "ip xfrm state". It's relatively
easy to do just by changing XfrmId in network byte order and
fudging XfrmUsersaId.

The pf_key test code also needs to be modified because it uses
the xfrm code to check the results. Unfortunately we can't just
fudge the pf_key SadbSa structure like we do for XfrmUsersaId
because the flag bits are spread across the flags integer as
opposed to being in only one byte of it. So we do have to use
htonl() there.

Test: xfrm tests pass on android-4.9
Change-Id: I44c9823c03a303a67514a3caee29a059dd6d0668