Album artwork returns triumphantly to the lockscreen.

It is pulled from the metadata of the active media
session, which we fetch in one of two ways:

1. From a proper media notification, we can extract the
   session token from EXTRA_MEDIA_SESSION.
2. If none of these is found, we ask MediaSessionManager for
   any outstanding media sessions whose packages match an
   active notification. These include legacy RCC clients,
   whose notifications are oblivious to EXTRA_MEDIA_SESSION.

We then pull the album artwork out of the metadata and fade
it in on the lockscreen.

Bug: 15147533
Bug: 15135254
Change-Id: Ia967bfba310b6c792b14673872cb132ddd7052a5