Refine GcMarker with specifics of finalization.

Rationale:
Rather than using a phantom reference on a sentinel
object, we use phantom references on the actually
tested objects, and wait for the right queuing
events to test subsequent finalized values.

Test: org.apache.harmony.jpda.tests.jdwp

Bug: 70958370
Change-Id: I24bbbca748b48beadf7d3c78b19a15ee935885e5