Add diffing to KernelCpuThreadReader

We want to not use WestWorld's diffing so that we can apply thresholds
on the diffs. This is further explained in the KernelCpuThreadReaderDiff
documentation.

Test: atest KernelCpuThreadReaderDiffTest
Bug: 129387487

Change-Id: I638ae92a28236830444dc80e470114a0e23d79a4