ServiceSingleton: prevent recaching an equivalent interface

Prevent spurious callbacks from returning a different
interface pointer representing the same binder object.

Flag: EXEMPT bugfix
Test: for run in {1..100}; do (sleep 8; echo $run; adb shell pkill audioserver); done
Test: atest service_singleton_tests
Bug: 386871946
Change-Id: Iae8f8b8108d0a9297d9678e060933cc74065a6be