Add battery power use reporting to batterystats service.

Move the BatteryStatsHelper class (which computes power use based
on the raw battery stats) out of the settings app and in to the
framework.  It is now used by batterystats dump output to print
the computed power information from its current stats.

This involved a lot of refactoring of the BatteryStatsHelper code
to remove all of the UI dependencies.  I also did a bunch of cleanup
in it, such as making all power computations be in terms of mAh.

Change-Id: I8ccf2c9789dc9ad34904917ef57050371a59dc28