[Bubbles] Ensure switch-in view visibility

For quick switches A->B->A, there is a chance that the animation
end callback of A->B happens after prepareForAnimateIn of B->A.
This will cause A to be invisible after the animation and the
bubble will disappear. This CL ensures A's visibility in
animation start callback of B->A to prevent such issues.

Bug: 403612574
Test: Manually switch between bubbles
Flag: EXEMPT bugfix
Change-Id: I032bdd3a1c7ac6977025fb4455724aa7eab994a1