Clean up MessagingLayout notifications

* Collapsed state now appears with the 'headerless' style used for all other collapsed notification templates (except conversations / calls which have their own style).
* Expanded state contnues to use the 'header' treatment of all our big states, but messaging faces will now appear in the left icon gutter, and text will always align with other styles.
* Image messages show up in the 'right icon' location when collapsed, but will not be moved to the 'left icon' when grouped.
* 1-1 conversations will highlight the other user's name as the title, in the collapsed state.

Fixes: 173204301
Test: post and inspect lots of messaging notifications
Change-Id: I5a56d11ea98be1a22caca0d85040ab3fc375216f