MediaSet: Rework to support sessions

Change-Id: Ie722d6255b0e2e3295eb2e6a2a6d571182e9e910
Implement methods to allow using sesions from rpcs

Change-Id: I9610869284b3a371dafce5ab4d5e862d34886498
ObjectRegistrar: Simplify code, now MediaSet accepts strings as id

Change-Id: Ib799db635eebd93c865aef1cd45fdb1e6095f8bb
MediaElementImpl: Make lines shorter

Change-Id: I59e85c4df645c445992566267e97add4c72f46bd