ART: Support per PID stack trace files.

Introduce an -Xstacktracedir argument that supplies a directory
under which stack traces are written, with a unique file created
per trace. The location of the actual directory in a production
system is still not decided, and follow up changes might be
introduced to supply a per process override.

Bug: 32064548
Test: test-art-host, test-art-target

Change-Id: If377ce6a2abe8b325f6441d8de222b1ea3f40ec9