Fix use-after-free asan hit in XfrmControllerTest

This test was forgetting to restore the original Syscalls object on
cleanup, causing an asan hit. The fix consists in switching to the safer
ScopedMockSyscalls, as all other tests do.

Test: atest netd_unit_test (with asan enabled)
Change-Id: I5ddaaf7e806aadd00b5b97006bf924ee5b7815dc
1 file changed