Phone: Add transparency to incall screen

Makes the background of multiview semitransparent.
It allows to at least see whole image of incoming caller.

Original commit: https://github.com/AOKP/packages_apps_Phone/commit/0a4c57816f55f6c0a1fbff457d2ccabc81e8f9e6

Change-Id: I920fee43e3bac7492a54f43281716eb8b686aa5e