Fix flaky ObjectReference.EnableCollectionTest

Moves test object allocations to a separate method to avoid keeping
references to these objects in local DEX registers and forces object
finalization by calling System.runFinalization method.

Also cleans up the construction of the message sent by the debuggee
to the test (indicating whether the test objects are finalized).

Bug: 21575091
Change-Id: Id23c5b04240090b9ff3b5bc9c1bce2614daeb4e8