call: don't show preview when camera is off

Change-Id: Idd9c91defdd792e537fd1e50797eb1f06d5f9ba6
diff --git a/ring-android/app/src/main/res/layout/frag_call.xml b/ring-android/app/src/main/res/layout/frag_call.xml
index beebdd0..fbc3d05 100644
--- a/ring-android/app/src/main/res/layout/frag_call.xml
+++ b/ring-android/app/src/main/res/layout/frag_call.xml
@@ -47,13 +47,15 @@
                 tools:visibility="visible" />
 
             <androidx.cardview.widget.CardView
+                android:id="@+id/preview_container"
                 android:layout_width="wrap_content"
                 android:layout_height="wrap_content"
                 android:layout_alignParentEnd="true"
                 android:layout_alignParentBottom="true"
                 android:layout_margin="12dp"
                 app:cardCornerRadius="16dp"
-                app:cardPreventCornerOverlap="false">
+                app:cardPreventCornerOverlap="false"
+                android:visibility="gone">
 
                 <cx.ring.views.AutoFitTextureView
                     android:id="@+id/preview_surface"