Ensure non null references for animateReorder() method.

Although I couldn't determine why BubbleController used
mBubbleStackViewCallback instead of mBubbleBarViewCallback, I've added a
safeguard to prevent the System UI from crashing with a
NullPointerException (NPE) during bubble traversal in BubblesStackView.

Fixes: 427284677
Test: Artificially injected nulls to the bubbleViews collection on the
caller side.
Flag: EXEMPT bugfix

Change-Id: I5639d8ecf2ee62f0d1cf4a5398f14f28645135a1