ART: Refactor IterateThroughHeap

Factor out callback. In preparation for extension API.

(cherry picked from commit f787fd3bc5476f73edb1722b4f6a09d52f69cd12)

Bug: 37283268
Test: m test-art-host
Change-Id: I0dc1a141afe43a415be9326bed24bd835dbbdc8c
1 file changed