ClatdTracker - do not store pointer to NetworkController

Test: build, atest libbpf_android_test libnetdbpf_test netd_integration_test netd_unit_test netdutils_test resolv_integration_test resolv_unit_test
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I677f40fa5f3694ecdc33be999996768e4e851480