Fix /data/anr/traces.txt can't be written by apps.

/data/anr/traces.txt is supposed to be created by ActivityManagerService
so non-system apps can write traces to it, but this wasn't happening.
It would end up being created the first time the system wrote something
to the file, leaving it unwritable.

(The file should really be 664, not 774, but since it's harmless let's
avoid the extra change from upstream.)

http://code.google.com/p/cyanogenmod/issues/detail?id=624