fdtrack: generate a heapdump when triggered.

It's somewhat hard to debug with just allocation backtraces when the
cause of the leak is a Java object leak. Generate an hprof dump when
this happens.

For privacy reasons, we don't include this in bugreports, since the
contents of system_server's memory is highly sensitive: they must be
manually requested from dogfooders.

Bug: http://b/176389805
Test: manual
Change-Id: Ib7755433395f341d90b732f285f00c8675291139