libaudiohal@aidl: Add missing HAL death handler

The death handler is used when the HAL gets stopped
using 'sys.audio.restart.hal' system property.
In this case the HAL is restarted using 'stop / start'
sequence, and as a result, 'onrestart' event is not
triggered (that's intentional).

Bug: 328569315
Test: adb shell setprop sys.audio.restart.hal 1
      then check if both audioserver and HAL get restarted
Change-Id: I7368557d2a8cb37d758882e94b64d6446aa9a671