Fix conversation list view for larger text sizes.

Reworked the conversation list item xml to fix various bugs, mainly for larger text sizes:
- Name text doesn't get drawn over the date text any more, but correctly abbreviated instead (all text sizes)
- Name text doesn't get drawn over the subject text any more (bigger text sizes)
- Date text now uses sp instead of dip, making it scale correctly.
- Fixed sizes of the Imageviews to match the name text (though they don't seem to be used yet?)

Image before:
![before](https://f.cloud.github.com/assets/1475672/2355699/2f0361f2-a5df-11e3-8e6f-1d3c45888c8d.png)

Image after:
![after](https://f.cloud.github.com/assets/1475672/2355701/33dd296a-a5df-11e3-88ec-a07ba2f0bffd.png)