Bouncer: Refine qs and notification alpha anim.

There was an issue that qs content was fading in with a delay. This is
because we are applying the bouncer interpolator in this instance. This
CL limits the scope of the bouncer interpolator to only when the bouncer
is in transit.

Bug: 227146048, 17409739
Test: Manual test on device plus unit tests
Change-Id: I398baa6d54f0c3aa51a9046dd822fc0acd33aadd