Add a new callback for the excessive binder proxy count

The callback is served as a warning where there are too many
binder proxies from a certain UID. The recipient of the callback
would then have a chance to log this event and/or trigger a dump,
before things getting out-of-control and trigger the high watermark
callback, where the process could be killed.

This callback signal could also be used as a hint to the GC in
the upper layer, which could effectively release the references to
remote binder stubs.

(cherry-picked from ag/25082740)

Bug: 298263955
Test: binder_bpBinderFuzz
Test: atest binderLibTest
Change-Id: I5d3659a9430c7404f7cfdfe4732980f5b1b0fa65
Merged-In: I5d3659a9430c7404f7cfdfe4732980f5b1b0fa65