Add a perf test for BatteryStatsHelper

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