AssistantMediaSession handle losing audio focus

1. Register AssistantManagerServiceImpl as a listener of libassisant media
manager and notify assistant_media_session about media metadata in
OnPlaybackStateChange().
2. Implemented addObserver(), suspend() and resume() API for
assistant_media_session.


CQ_INCLUDE_TRYBOTS=luci.chrome.try:linux-chromeos-chrome

Change-Id: Ib81b9b10e1fad6e711e48b31f0703dd710ae0ba6
Bug: 130315881
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1557006
Commit-Queue: Xiao Yang <yanxiao@google.com>
Reviewed-by: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652747}
4 files changed