CrOS GMC V2: Fix bugs for adding cast media items to updated CrOS media
view

- Make CastMediaNotificationProducerKeyedService consume
MediaItemManager from both MediaNotificationProvider and
QuickSettingsMediaViewController and create
CastMediaNotificationProducer for both so that the quick settings view
will display cast media items too

- Fix the logic for hiding the media item that has presentation of
another casting media item; only need to hide it in
MediaSessionNotificationItem which will always be called; do not hide
the media dialog since there can be other media items displaying

- Add the "Stop casting" button in MediaNotificationViewAshImpl for
easier testing though the UI needs further work

- Fix the crash of ~MediaNotificationViewAshImpl()

Bug: 1442056
Change-Id: I9b821fb1357173c578d43bec4d353b25c64e6e99
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4606305
Commit-Queue: Yiren Wang <yrw@chromium.org>
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Reviewed-by: Andre Le <leandre@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1157288}