bootctl: pass cookie to death recipient

On binder death, the cookie is the value that is passed to the death
recipient. This value shouldn't be nullptr here since our onDeath()
function expects an instance of BootControlClientAidl. If that doesn't
exist, we have a null pointer dereference which will cause update_engine
to crash.

Bug: 369289491
Test: crash bootctl service
Change-Id: I0bed4680c23b7d3516ed43aa566c4474962244fa