Monitor CPU availability using CPU frequency stats.

- Implement the core CPU availability monitoring logic by reading
  the CPU infos with CpuInfoReader.
- Start debug monitoring on user-debug and eng builds when no
  callbacks are registered.
- Update CpuAvailabilityInfo to cache the uptime of the cached data.
- Update CpuAvailabilityMonitoringConfig to return cpuset string.

Test: atest CpuMonitorServiceTest
Bug: 242722241
Change-Id: I6e12c11f243966914079f54df4430d69b797ec1f