Slightly modify Selectable.is{Hidden|Disabled|Selected}In default return value.
Slightly modify Selectable.is{Hidden|Disabled|Selected}In default return value.

If a model does *not* implement Selectable, usually user wants it to mean "isSelectable" - i.e. this is a signal that everything is selectable. On the other hand, if a model does *not* implement Selectable, the user does not want "isDisabled" or "isHidden" to automatically be true.

PiperOrigin-RevId: 205551262

angulardart/angular_components,angulardart/angular_components,angulardart/angular_components