Use Map inside DrmEngine
Use Map inside DrmEngine

Updated DrmEngine to use Map internally. Object is still returned to help
limit the size of the change.

Change-Id: I7e2b5aa5a8bce58d5a6d09a73c2926869d37d906

shaka-project/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player