Read per-process GPU memory when pulling ProcessMemorySnapshot

This is an eBPF usermap read via libmeminfo. System server
already has permissions to do so (as it already does so for the
total GPU mem stats).

Change-Id: I9e3c4dc9144708c630215e309b7ff394abbdcd76
Test: manual, statsd_testdrive 10064
Bug: 208440056