Add avatar support.

Add avatar support.
Fixes second wear page to start scroll at bottom, like Hangouts.
Cleanup notification builder, extend once.

![image](https://cloud.githubusercontent.com/assets/4513759/4435406/080e30e4-4743-11e4-93aa-d8a9395622f5.png)
![image](https://cloud.githubusercontent.com/assets/4513759/4435409/26f1b7ce-4743-11e4-9d68-9fd5444a864b.png)

Not sure about the avatar scaling on the notification bar yet. I retrieve the 400x400 avatar, as recommended by Google for wear backgrounds, but it doesn't look like it scales down for the largeIcon as I figured it would. Will look into that next.

Also, using in memory cache for the avatar, to avoid adding the extra permission for WRITE_EXTERNAL_STORAGE.
It's actually not because of that, oddly enough. That just specifies the scaling behaviour, and that it should keep the exact ratio.

However, doesn't seem like it actually gets scaled down for the notification, so I need to figure out how to downscale it without downloading it a second time. Looking into why this happens tonight.