New Kernel Per-UID CPU Time Readers

Kernel per-UID CPU system & user / concurrent active / concurrent
policy / per-frequency time readers that reads the string version
of the per-UID CPU time proc files via the new
KernelCpuProcStringReader.

They are all subclasses of and nested under KernelCpuUidTimeReader,
for a cleaner directory organization.

Bug: 111216804
Test: 4 Unit Tests
Change-Id: I00836c02fff639245c4285b5edaf7b4c94611405