ANDROID: uid_sys_stats: defer process_notifier work if uid_lock is contended

process_notifier() is called every time a process exits. When multiple
processes exit roughly at the same time, the uid_lock taken from inside
of process_notifier() will create contention which slows down process
exit. Defer stats accounting in such case to avoid lock contention.

Bug: 261537194
Change-Id: Ia1e9a451eab39eb0dda7eb175bfd71c67f3e0a58
Signed-off-by: Suren Baghdasaryan <surenb@google.com>