Use reference equality for AnimatableScaleMatrix equals().

When we recreate the stack, we also create a new AnimatableScaleMatrix and get a PhysicsAnimator instance for it. Since the matrices can have identical values, we sometimes end up getting the animator instance for the previous stack's matrix if it hasn't yet been garbage collected. This means we're animating a matrix that has no effect on the current stack, so the expanded view scale stays at 0% forever.

Bug: 159719888
Test: this one is really hard since it's nondeterministic, but just create/dismiss bubbles a lot
Change-Id: Ib5e390caba0ac621ccf7a1da71adc9d15f51fd65