Use handles for GetReferringObjects

Required since VisitObjects may cause thread suspension. Fixes
failing jdwp tests.

Test: test-art-host
Test: art/tools/run-jdwp-tests.sh '--mode=host' '--variant=X32' --debug

Bug: 31113334
Change-Id: Ic11780b3778c83e7812bc90c0723a293537f376c
3 files changed