BusyIndicator: Fix initialization of the running property

plasmashell can trigger constant repaints because of invisible busy indicator animation.

The current logic relies on the fact that the RotationAnimator does not run by default. However, it's not the case when using the "on" syntax and the animationRunningChanged signal may not be emitted when the contentItem is constructed. Therefore, the running property may not be set to "false" even though the busy indicator is invisible.

This change refactors animation code a bit to ensure that RotationAnimator.running is always initialized to a correct value.