ahat: show sample path through strong references.

Instead of showing a sample path through weak references, which is
much less useful for debugging why the object is retained.

Change-Id: I2b8d8bc9368cdcabd7a369e45278ba9b44a38e40
Bug: 64592321
Test: m ahat-test, with new gcRootPathNotWeak test added.
3 files changed