Initial commit of the libcore heap metrics test.

This CL adds an activity that dumps heaps in a controlled fashion, and
a host-side test that launches the app, collects the dumps, and
exports some metrics derived from them.

This is a starting point for further work.

Bug: 67999970
Bug: 67283029
Test: tradefed.sh run commandAndExit template/local_min --template:map test=libcore-memory-metrics-tests
Change-Id: I836903c030c894274faf17ba0e0e36771d68dba0