Treat thread state kWaitingWeakGcRootRead as kJavaRunnable.

Like kSuspended, kWaitingWeakGcRootRead means temporarily waiting for
GC's purpose rather than the thread is waiting on a monitor.

This may fix some jsr166 test failures.

Bug: 25883050
Bug: 12687968

Change-Id: Ib53e18c0fb2ff95aa0491b1531201e583288684e