Add a new VtiInterface class and use it in XfrmVtiTest.

This class encapsulates both the knowledge needed to create a
VTI interface and the state that it holds. This will be useful
in future changes to store multiple VTIs in the same test.

Test: xfrm tests pass on android-4.9
Change-Id: I5b047a41e384b94b7cf50546026b32081f5d00bd