[Media Image] Move image observer to separate call

This moves the image metadata to a separate call on
the MediaSessionObserver and adds a MediaSessionImageType
to group different images by type.

We are moving it out because MediaMetadata is used
in both the session and controller and we want to
add image handling to the controller (so clients do
not need to worry about MediaImage).

BUG=928369

Change-Id: I68433ec82d71c81a63a8183679a1b365ea13f606
Reviewed-on: https://chromium-review.googlesource.com/c/1452467
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Sean Topping <seantopping@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631522}
10 files changed