Once background and resuming is supported, the ads loader will be kept when the
player is destroyed and recreated. Move the state relating to the structure of
ads and what ads have been loaded/played out of the media source and into the
loader so the information is not lost when the source is released, in
preparation for supporting background and resuming.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=161503571
ef56c9fe39