BHV-6832: Fire onEnd when rAF animation has actually finished.

Issue
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The `onEnd` event is fired before the animation has actually finished, which can have a negative impact on animation performance if there are complex operations that are run in the `onEnd` handler.

Fix
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We make sure `onEnd` is sent after the final animation step, via `enyo.asyncMethod`.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>