Check if CPU time tracking is expected to work

Register the puller for CpuTimePerClusterFreq only if CPU time tracking
is supported. This avoid passing all zero times around and recording
"no data" state.

Depends on https://r.android.com/1566193

Bug: 174245730
Test: cmd stats pull-source 10095
Test: atest CpuStatsTests
Change-Id: Ifb3da84711b111eabc4af7a166dec794278b4a02