Choose thread icon more carefully
b/17298161
Choosing the avatar to draw as the thread icon follows this algorithm:
1) Prefer the sender of the first unread message.
2) If all messages are read, prefer the last sender that is not the
current account.
3) If all messages were sent from the current account (e.g. user is
emailing themselves), use the last sender (aka current account).
In the process of doing this work the last remaining dependency on
DividedImageCanvas was broken, so it could be removed at this time.
Tests confirming the new behavior have been added to SendersFormattingTests.
Change-Id: If8e066de8cb98f2f95b019a88a2fdadc2f9f5090