SDK / exoplayer

The DefaultMediaNotificationProvider caches the last loaded artwork
bitmap so that the bitmap isn't loaded again when the notification is
updated, e.g., the player is transiting from playing to paused. However,
loading bitmap requests for bitmaps that are already being loaded are
not suppressed. For example, if the notification is updated while the
artwork is still downloading, the same artwork might be downloaded
multiple times.

This change suppresses a bitmap load request if the same artwork is
still being loaded, to avoid additional artwork downloads.

#minor-release

PiperOrigin-RevId: 462572221
(cherry picked from commit dd2c16bc)