Control dialer's incoming call proximity sensor check via an overlay

This feature can't be used on devices with proximity sensors that don't
work when the screen is off, e.g. sensors using ultrasound technology.
The result in those cases is a black screen with just the status bar
showing, instead of the normal incoming call UI.

Make it possible to opt-out via an overlay.

Change-Id: Ic3848d09e1ed80e5409cbecbaca2517db16ed0b8