Only fillLowPowerStats when needed

This fixes a boottime regression.
Prior to fix, FinishBooting took ~900ms to complete
After fix, FinishBooting took ~120ms to complete

Fixes: 193074223
Test: atest google/perf/boottime/boottime-test
Change-Id: If85a93f1f92c8af8251c870bf576bb125778b011
1 file changed