Simplifies view-binder.

The view-binder no longer has the view-model-like logic to combine the
UI state of the button with the "animateReveal". Instead, the view-model
takes care of it, simplifiyng the view-binder to make it a bit simpler
and move the complexity into the view-model which allows us to test
that.

Bug: 235403546
Test: Unit tests. Manually verified appear animation is still happening.
Change-Id: Ic836b0e1133d69af0726a4d7fcd4553fd532231f