Repeat proxy limit callbacks if we keep going above the limit.

When we hit the high watermark for number of live proxies in a process,
we call a callback with the offending UID. ActivityManagerService will
then kill all processes belonging to this UID. The proxy limit callback
is only "re-armed" when the number of proxies for that UID drops below a
certain value again. However, because the proxies beloging to the now
dead processes will only be removed when the GC next runs, and restarted
processes may start adding new proxies in the mean time, we may actually
never drop below the lower threshold again, and never fire the callback
again, despite an ever-increasing amount of proxies.

To prevent this, repeatedly fire the limit callback every time that we
go another # of high watermark proxies over the limit. This, in
combination with a GC in the framework, should make sure that we
regularly kill and clean up these processes.

Bug: 198340142
Test: Manual
Change-Id: I8d877d409863fdae9cffd43709b647d0c1edb440
Merged-In: I8d877d409863fdae9cffd43709b647d0c1edb440