Fix sound pileup issue

[docEdub]

Sounds created with the loop or autoplay options set while the audio engine is locked are started automatically when the audio engine is unlocked. If the newly created sound is stopped before the audio engine is unlocked, then the sound still plays when the audio engine is unlocked. This is unexpected. Stopped sounds should not play when the audio engine is unlocked.

This change fixes the issue by clearing the 500 ms timeout used for double-checking the audio context state and removing the observer added to AudioEngine.onAudioUnlockedObservable when the sound stops, pauses, gets played again without being stopped, or when it gets disposed.

Reported on forum https://forum.babylonjs.com/t/sound-pileup-in-chrome/42266.
Tested on Edge browser with playground https://playground.babylonjs.com/#0CNZFE#21.

[sebavan]

Should the cleanup code not be called in Play as well ?

[bjsplat]

Snapshot stored with reference name:
refs/pull/14047/merge

Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14047/merge/index.html

To test a playground add it to the URL, for example:

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14047/merge/index.html#WGZLGJ#4600

Links to test babylon tools with this snapshot:

https://playground.babylonjs.com/?snapshot=refs/pull/14047/merge
https://sandbox.babylonjs.com/?snapshot=refs/pull/14047/merge
https://gui.babylonjs.com/?snapshot=refs/pull/14047/merge
https://nme.babylonjs.com/?snapshot=refs/pull/14047/merge

To test the snapshot in the playground with a playground ID add it after the snapshot query string:

https://playground.babylonjs.com/?snapshot=refs/pull/14047/merge#BCU1XR#0

[docEdub]

Should the cleanup code not be called in Play as well ?

Yep, thanks. Unit test added for this case, too.