Read current CPU frequencies and populate CpuInfo.

- CPU time in state file provides information on the time spent by CPU
  cores in a specific frequency since system bootup. This time is
  monotonically increasing since system bootup.
- Use the time in state information to calculate the average frequency
  of a CPU core between two polling periods.
- The unit tests require test files to read the CPU info. The test files
  in the test APK are not accessible by the implementation. Thus, copy
  the test files from the APK into the test app's cache directory during
  setup and remove them from the cache directory during teardown.

Test: atest CpuInfoReaderTest
Bug: 217422127
Bug: 242722241

(cherry-picked from commit c1057fef03d755e35e6d6723480dc7a9cbbeb3c8)

Change-Id: Iadb234a56b48dcba3c8dc77411b175de5046ce89