Change DecodeWeakGlobal to DecodeJObject for class unloading

DecodeWeakGlobal returns the sentinel object for cleared JNI weak
globals. This was causing a memory leak since it wouldn't delete
class tables and linear allocs due to never returning null. The bug
was found by yamauchi.

Bug: 22720414

Change-Id: Iff4681495232b9a9756dbdb51d10ea72691a85dd