Add atom to StatsCompanionService for retrieving per-thread CPU usage data

N.B.: This calls CpuThreadProcReader synchronously in
StatsCompanionService::pullData. This call takes approximately 50.079ms on a
Pixel 2.

Bug: 111534779
Test: `adb shell cmd stats pull-source 10035` returns correct data

Change-Id: I6fe6d178e28669f10ba9c076cbf19dc443d171c9