Bring back change from #2016 (Don't set initialized flag to false in MediaPlayer.reset() )
Created by: dpieri
This intended change from #2016 was reverted as part of a refactor https://github.com/Dash-Industry-Forum/dash.js/pull/2007 (https://github.com/Dash-Industry-Forum/dash.js/commit/3abca12a1eeb0df89224d4676d6089b4e64d9485)