[media][audio_core][e2e] Tweaks to support virtual audio

This CL makes the following surgical changes to prepare AudioCore
for the usage of VirtualAudio technology, in support of the e2e push:

* Includes error codes with ShutdownSelf calls (previously 6 of 21
  call sites didn't),
* Handle AUDIO STREAM_PLUG_DETECT_NOTIFY as other messages are (as
  opposed to hard-coding the assumption that it is identical to an
  AUDIO_STREAM_CMD_PLUG_DETECT response),
* Accept (but ignore) AUDIO_RB_POSITION_NOTIFY messages, rather than
  shutting down the stream when these are received,
* Don't log the initially-configured format, as each new device is
  detected and initialized. The debug log chatter as these devices
  come and go was reducing the usefulness of the logs overall. A
  TODO suggests that we store this info in the hub, if it something
  worth knowing later.

AudioCore does NOT request ring-buffer position notifications from
drivers during streaming; this is unchanged. Instead, it relies upon
CLOCK_MONOTONIC as the source of chronological truth.

Test: build, CQ

Change-Id: I6b8b4b0186999bb22e9e855ec2e69efab48bdcac