Don't require XFRM tests to flush all state on tearDown.

This will allow tests to set up XFRM state and policies in
setUpClass instead of in setUp. This speeds up running the tests
because setting up iptables rules can take a couple of seconds.

This is also useful when running on device, where blowing away
XFRM state in tests could be disruptive to other system
functionality.

Bug: 70371070
Test: xfrm tests pass on android-4.9
Change-Id: I6246c172368fd65a815175ecf58b53965de1bcfd