Report post-GC memory metrics for system_server

Currently, applications register their callbacks to report post-GC
memory metrics in ActivityThread.handleBindApplication. This however
does not apply to system_server. This CL registers the callbacks
specifically for system_server to report the post-GC memory metrics.

Also, change to use `Process.myProcessName()` for the process name
as `Application.getProcessName()` does not work for system_server.

Bug: 382022031
Flag: android.app.report_postgc_memory_metrics

Change-Id: If3345d9db4ce26743d39b0a17c9092ce23b7ffcd