optimize nullbooleanfields for mobile (bug 917439)

**BEFORE**

![screen shot 2013-09-17 at 11 48 19 am](https://f.cloud.github.com/assets/674727/1159633/c0abf99e-1fc9-11e3-9bc9-cb8d13b9345a.png)

**AFTER**

![screen shot 2013-09-17 at 11 47 48 am](https://f.cloud.github.com/assets/674727/1159635/c1e58e60-1fc9-11e3-9faf-bc3448664962.png)