Fix Tunnel Encryption with New Scapy

New Scapy requires that all parts of a packet be
objects rather than just raw bytes. Thus, the
various payload-building pieces of the encryption
process now need to be assembled via scapy.Raw()
elements, which still resolve to strings when
str() is called on them, but behave nicely when
building packets using Scapy's methods.

Bug: 72717984
Test: xfrm_test.py on common 4.4, wahoo, and sailfish
Change-Id: I56decc323f1e5a15478a6a6d5caff4e92656fc44