Optimize set_sched_policy(), which gets called in every binder call.

- use static /dev/cpuctl filenames, rather than sprintf() on the fly
- use a custom formatter instead of sprintf() for the tid number

This is a simplified version of Dan's original
Ifc9c81f74fe65f0695ead53b30194bc6adf00da0 to be less risky for Froyo.

Bug: 2660235
Change-Id: I744bbc2ec01fd2569612c814a497df40a933b622