Clean up KernelCpuThreadReader

This includes (as well as nits by IntelliJ):
- Removing getCurrentProcessCpuUsage, as it was only used in tests
- Updating class documentation
- Adding @Presubmit annotations to tests
- Reformat using google-java-format

Test: atest KernelCpuThreadReaderTest

Change-Id: I710773043b9e9ef40545c20667fe2f79e9915e43