Stop observing death of PrintManager listener when it is removed

Symptom:
Global reference table overflow happens on system_server.

Root cause:
When a listener is added to the PrintManager, death notification for
the listener is registered. So the listener is added to global
reference table.
But the death notification is not unregistered when the listener is
removed from PrintManager. The listener isn't removed from the global
reference table until client process die. If client process repeatedly
add/remove listener, the global reference table overflow.

Solution:
Call unlinkToDeath() when the listener is removed from PrintManager.

Test: cts-tradefed run cts-dev -m Print
Bug: 68746445
Change-Id: I0b133e4607317a15e51bdf6e1248552dcc920d6d