Improve efficiency for CpuTimePerUidFreq

Tested on marlin. Shortly after boot, the total is 468 fileds with non
zero time spent at frequency.
It quickly swell to 1600+ though.
It is some improvement compared to 2700+, but still an extremely large
pulled atom.

Bug: 74965273
Test: manual test
Change-Id: Ibbeaa02f54d91c4958d75dd3aaafbb306d61f8d8