Android: Mark mark as read icon bigger

Fixes #280. Replaced with the icon suggested by Samuel. The result is shown below. Tested on my HP Touchpad running 4.0.4 and my ZTE Blade phone running 2.3.

![screenshot_2013-06-05-22-52-38](https://f.cloud.github.com/assets/1684641/614978/42be2fc0-ce2a-11e2-8b15-a36011ce9798.png)