App & channel notification settings updates

- change 'block all' to an on/off switch bar
- update channel list summary text when notifications are
toggled on/off on app settings page
- Add 'off state' text
- change style of foot items
- change 'importance' from a dropdown to its own page

Bug: 37538972
Bug: 37479730
Fixes: 37541624
Fixes: 37549732
Test: manual

Change-Id: I0e5cc66ba539ce2b76b4ad6541bf6bfb5b58c373

res/xml/app_notification_settings.xml

@@ -18,13 +18,6 @@
         xmlns:settings="http://schemas.android.com/apk/res/com.android.settings"
         android:title="@string/app_notifications_title"
         android:key="app_notification_settings">
-    <!-- Block -->
-    <com.android.settingslib.RestrictedSwitchPreference
-        android:key="block"
-        android:title="@string/app_notification_block_title"
-        android:summary="@string/app_notification_block_summary"
-        settings:useAdditionalSummary="true"
-        settings:restrictedSwitchSummary="@string/enabled_by_admin" />
 
     <!-- Show badge -->
     <com.android.settingslib.RestrictedSwitchPreference