sepolicy: restrict access to uid_cpupower files

Do not let apps read /proc/uid_cpupower/time_in_state,
/proc/uid_cpupower/concurrent_active_time,
/proc/uid_cpupower/concurrent_policy_time.

b/71718257

Test: Check that they can't be read from the shell
    without root permissions and system_server was able
    to read them

Change-Id: I812694adfbb4630f7b56aa7096dc2e6dfb148b15