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
diff --git a/server/XfrmControllerTest.cpp b/server/XfrmControllerTest.cpp
index daa7b75..5b90860 100644
--- a/server/XfrmControllerTest.cpp
+++ b/server/XfrmControllerTest.cpp
@@ -129,10 +129,8 @@
 }
 
 class XfrmControllerTest : public ::testing::Test {
-public:
-    MockSyscalls mockSyscalls;
-
-    void SetUp() override { netdutils::sSyscalls.swap(mockSyscalls); }
+  public:
+    testing::StrictMock<netdutils::ScopedMockSyscalls> mockSyscalls;
 };
 
 // Test class allowing IPv4/IPv6 parameterized tests.