Run a GC after killing a UID due to binder proxy limit.

After we hit the per-UID proxy limit and kill the originating process,
there's actually no guarantee that the proxies to the (now dead)
processes are cleaned up immediately; if the processes get restarted and
start accumulating proxies again, we may go over the global proxy limit
(20k currently). Do a GC to make sure we clean them up.

Note that the GC here might not actually clean up all the proxies,
because the binder reference decrements will come in asynchronously; but
if new processes belonging to the UID keep adding proxies, we will get
another callback here, and run the GC again - this time cleaning up the
old proxies.

Bug: 198340142
Test: Manual
Change-Id: I75c7ea4f7aa4a5d448f610014770f1b9788cfb5b