call: fix progress animation

El Capitan handles animations differently and optimizes out
animations on off screen views. Progress animation was started when
Ring was launched, and the CurrentCall view is hidden at first,
therefore progress animation was never started.

This patch also simplifies layer handling.

Change-Id: I33f1b42ca3e813d780868a81f4e391138e4751a2
Tuleap: #269