Add test for death notification workqueue.

Adds a test to makes sure death notifications for
nodes that are already dead are pushed to the binder
threadpool, and not the thread that is registering
for them.

Bug: 23525545
Test: builds, passes (with kernel fix).
Change-Id: I6b4d6ae99d7a4daa75ca32619f28e93595c60490
Merged-In: I2f43a0fa828877104bbf9b8c33a80b5199225b3d