MeasuredEnergyStats supports custom energy buckets

MeasuredEnergyStats now supports a variable number of
'buckets', including both the standard buckets (fixed number)
and additional custom buckets (variable number) whose meaning
is unknown to BatteryStats (but is enumerated as an 'ordinal').
This will be used to store per-uid energy data from custom
power components.

Test: atest MeasuredEnergyStatsTest
Test: atest com.android.internal.os.BatteryStatsTests
Bug: 179107328
Bug: 174186358
Change-Id: I0022ac6940daca5f1e7d0e502eed01effa71f796