On Bramble, we get these results:
endToEnd_median: 321.983605
getStats_median: 258.134786
getStats_cached_median: 48.506614
powerCalculation_median: 51.798291
Test: Lock the device to little cores for consistency
$ ./frameworks/base/libs/hwui/tests/scripts/prep_generic.sh little
$ atest BatteryStatsPerfTests
Bug: 173446001
Change-Id: I307bcea38c7001d950de1b506c31b64f89629952