SliceLiveData should remove observers.

As positions of items will be changed, new sliceViews will be created.
SliceLiveData has been set to observe a certain old sliceView. When the
page is re-launched, we cannot garantee we always get the same
sliceView, which means the sliceLiveData may be still observing a sliveView that
is no longer applicable. This will lead to broken slices.
Hence, we need to remove observers first then make it observes a new sliceView.

Bug: 117922624
Test: robotests, visual
Change-Id: I59368a6ce5fa84487c9a4788b970fdc074a0c2a9