[Android MR] Fix several issues in the CAF implementation

This CL includes misc fixes:

* Volume change: when the volume changed, broadcast `update_session`
  message before acking requests.
* Join session: fixes the join session flow. Pending AI is to handle
  `leave_session` properly.

Bug: 711860
Change-Id: I3f66121ffb188b5c3afd53bab5db7bce98089c89
Reviewed-on: https://chromium-review.googlesource.com/1228138
Commit-Queue: Zhiqiang Zhang <zqzhang@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591880}
5 files changed