Move some utility code from xfrm_base to xfrm.

This change moves UserPolicy, UserTemplate, and CreateTunnel from
xfrm_base.py to xfrm.py. These methods are now generic enough to
belong there, and putting them there allows them to be accessed
from code that does not subclass XfrmBaseTest, such as the
upcoming VtiInterface class.

Bug: 70371070
Test: all xfrm tests pass on android-4.9
Change-Id: I4fb452690c23c398873fef363eeba17e87826af2