Fix race condition in WeakHashMapTest.

There was a race condition where the reference queue daemon wouldn't
necessarily have processed the cleared references by the time we
called WeakHashMap.size(). The race is fixed by doing a busy wait
with a 5 second timeout. This should't increase the test time in the
average case since the cleared weak references should be processed
shortly after induceFinalization finishes.

Bug: 15775475

Change-Id: I66350b6c87252e98bfbd945fe1f96bef58ec24a9