Don't override alpha for invisible elements when squishing

The seekbar is set to 0.0f alpha when disabled, which was getting
overridden to scale between 0-1 when transitioning between views.
Instead, if an element is invisible in a view state, keep the alpha at 0.

Fixes: 313908304
Test: visual
Test: atest MediaViewControllerTest
Flag: NONE
Change-Id: I70a552efe35fdbe90cd011576d1bd38442472220