Conversations UX polish pass

  - adjust position of expanded group avatar to remain vertically
    centered with title

  - adjust top padding of guts to match that of conversation layout

  - vertically align app name and time in the conversation header

  - clean up some of our XML styles

Fixes: 157757386
Test: manual, visual
Change-Id: I5eb774b44ce641a97c5deb706711f3b252e6554e