Add Reference.refersTo()

... and use it in ThreadLocal to avoid danger of blocking on GC
if it's at a stage at which we cannot safely convert WeakReferences
to strong ones.

Add test for refersTo.

Test: atest ReferenceTest
Bug: 172573708
Change-Id: I85b9701433f990684e7bd64afb8fc80abbf9b3ba
3 files changed