GitLab

In audio focus there is a concept of an "active"
media session. This is the top most non-temporary
session. This is slightly different from the current
focused session which may be temporary. We use the
active media session for media controller too.

This adds a method for tracking the current active
media session. This will be used for the media
notification for hiding/showing it. At the moment
it uses focus which may result it being shown
incorrectly.

BUG=897836

Change-Id: I0e2f82ffe23394e04d72622aa0d27cd4e8834d7a
Reviewed-on: https://chromium-review.googlesource.com/c/1330696
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608153}