Off-device library for the power model.

This first CL adds a class, PowerProfile that parses the power
profile xml file into a set of individual *Profile classes, one
for each of the hardware "components."

There will be more to come.  This library will be used to compute
the power model from a batterystats or statsd dump, with abstractions
so clients don't need to know all of the nuances of batterystats'
old versions, or statsd's configs.

Test: atest frameworks/base/tools/powermodel --host
Change-Id: I79802f91234b09539072d10f15534cef391fe04a