AudioPlayer: Stop using Object.observe() and Array.observe().

Object.observe() and Array.observe() will soon be deprecated.
This CL modify audio player to stop using it by listen to AudioPlayerElement's
property changes directly, rather than the model object.

BUG=552113
TEST=manually

Review URL: https://codereview.chromium.org/1495873002

Cr-Commit-Position: refs/heads/master@{#363425}
7 files changed