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.