Convert simple abr manager unit to ES6.
Convert simple abr manager unit to ES6.

Issue #1157

Change-Id: I55a732a76d89217b677c375d4000fef2947018ae

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