Stop AdCounter timer on element release.
Stop AdCounter timer on element release.

We did not stop the AdCounter element's timer when it was released
before. This meant that changing the controls while an ad was playing
would cause errors.
This adds an override for release to AdCounter, that does that.

Change-Id: I07e4f9c09c466d8130f31a35ceae70df622f3da2

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