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