overviewControls: Fix initial slide-in animation

overviewControls: Fix initial slide-in animation

Controls are slided in by animating translationX from the actor's
width to 0; however as _updateTranslation() will skip the animation
when the property is already at its target value and 0 happens to be
the initial value of translationX, the initial animation is skipped.
Fix this by initializing translationX to undefined, which will always
differ from a valid target value.