Zen: Move zen mode preference to switch bar.

- Add summary line support to SwitchBar.
 - Remove obsolete strings.
 - Wire up switchbar to zen mode w/ dialog prompt.
 - Remove obsolete callback pref helper.

Bug: 20064962
Change-Id: Ifede00b5d43d441ccd94db96bd2796bc57d1a990

res/layout/switch_bar.xml

@@ -24,6 +24,8 @@
               android:layout_width="0dp"
               android:layout_weight="1"
               android:layout_gravity="center_vertical"
+              android:maxLines="2"
+              android:ellipsize="end"
               android:textAppearance="@style/TextAppearance.Switch"
               android:textColor="?android:attr/textColorPrimary"
               android:textAlignment="viewStart" />