Keep track of DeathMonitor cookies

This change keeps track of the objects that the cookies points to so the
serviceDied callback knows when it can use the cookie.

Test: atest neuralnetworks_utils_hal_aidl_test
Tets: atest NeuralNetworksTest_static
Bug: 319210610

Change-Id: I418cbc6baa19aa702d9fd2e7d8096fe1a02b7794