Affordances no longer clickable when invisible.

This CL fixes a bug where the user can click on the invisible (alpha =
0) quick affordances on the lock-screen. The reason they were clickable
   was because the affordance views are fully faded-out, not truly invisible
   or gone.

The approach taken is simply to make the views not clickable as soon as
they start being faded out.

Fix: 241830987
Test: manually verified that the buttons are no longer clickable when
the screen is locked and the notifications shade is pulled down (which
is how the manual test failed) and added unit tests for the view-model.

Change-Id: Ic8f6305cef2b544fc468290cd56fc381c48e0877