Allow only system_server to read uid_time_in_state

Bug: 62706738
Bug: 34133340
Test: Check that uid_time_in_state can't be read from
the shell without root permissions and that
"dumpsys batterystats --checkin| grep ctf" shows frequency
data (system_server was able to read uid_time_in_state)

Change-Id: Ic6a54da4ebcc9e10b0e3af8f14a45d7408e8686e