AudioService: Add the AudioDeviceBroker's message queue to the dump

Clogged queue means device-related requests will not be served.

Example dump snippet:

AudioDeviceBroker:
  Message handler (watch for unhandled messages):
    Handler (com.android.server.audio.AudioDeviceBroker$BrokerHandler) {753fed2} @ 359741
      Looper (AudioDeviceBroker, tid 115) {ed3aba3}
        (Total messages: 0, polling=true, quitting=false)

Bug: 135763139
Test: adb shell dumpsys audio
Change-Id: If2130a4bd8bd9536df01656fe1a8f94548912ab9