Correctly use handles for JVMTI heap iteration.

We were incorrectly using an ObjPtr to store the filter-class in
IterateThroughHeap and FollowReferences. This could cause issues if GC
was occurring during the early parts of the call. Fix this issues by
properly handlerizing the pointer.

Test: ./test.py --host
Bug: 161574896
Change-Id: I2ed8e3e4b7af8fded69e8d86adf2049e907289e8
2 files changed