fix(ads): Fix IMA crash when autoplay is rejected (#4518)
fix(ads): Fix IMA crash when autoplay is rejected (#4518)

A preroll ad fails to start when there is no autoplay. This happens
because Shaka doesn't check if playback is allowed before calling the Ad
Manager's `start()` method. IMA's
https://github.com/googleads/googleads-ima-html5/tree/main/attempt_to_autoplay
addresses this by resolving a `play()` promise.

This waits to start AdManager until a `play` event from the video
element.

Closes #4179
shaka-project/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player