Fix system_server crash while iterating CookieTagMap

When uid is removed, NetworkStatsService will iterate CookieTagMap and
delete the corresponded uid socketTag value. But not only
NetworkStatsService would detete CookieTagMap entries. There are other
threads may also delete CookieTagMap entries such as #unTagSocket in
bpfHandler and desctroy socket listener in TrafficController. System
server crash as NPE because the CookieTagMap entry it iterate just be
removed by other threads at the same time. This is just a simple fix to
prevent the crash. Will have follow up CL for formal fix.

Bug: 220084230
Test: m
Change-Id: I08cad87f537fb09499faf1ff5cfd443fcb8ce436