Rewrite KernelSingleProcessCpuThreadReader to native

For now, the native implementation is identical to the
Java implementation.  The performance improvement is ~20%
in a microbenchmarks.

Bug: 169279846
Test: atest FrameworksCoreTests:com.android.internal.os.KernelSingleProcessCpuThreadReaderTest
Change-Id: I8685a7f0adf9476209e29bb3ee3e89e99f66fa95