Avoid reading cpuinfo_(max|cur)_freq stats in CpuMonitorService.

The cpuinfo_cur_freq file read access is available only to the root
user. Android framework cannot read this file. So, do not read stats
from the cpuinfo_*_freq files.

Test: atest CpuInfoReaderTest
Bug: 263154344
Change-Id: I45263935fae844b4d9d1b6ec50876cc34c95a09a