Fix PowerHAL cpufreq permissions

The process containing PowerHAL didn't have group permissions to open
/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor|max_freq|min_freq.

Modified permissions to match AOSP using the system user as the owner.

Change-Id: Idbdad8b00b8ad7d7d2a845287cccbabb58fc5aba