Add some detailed data as part of battery history.

Every time the battery level changes, a new extended
detailed use data structure is written to the history.
This currently only contains delta CPU use since the
last detailed entry (total CPU and to three uids), but
it gives us the infrastructure for adding more detailed
data in the future.

A detail entry for regular history looks like:

     Details: cpu=15730u+2307s (u0a57=11312u+502s, 1000=2344u+701s, 0=473u+991s)
              /proc/stat=15377 usr, 1797 sys, 197 io, 0 irq, 8 sirq, 23103 idle (42.9% of 6m 44s 820ms)

u is user-space cpu time, s is system/kernel time.

The equivalent check-in output is:

9,h,0,Dcpu=15730:2307/10057:11312:502/1000:2344:701/0:473:991
9,h,0,Dpst=15377,1797,197,0,8,23103

Also add a new "unplug" command to the battery service,
to easily put it into a state where it always considers
the device to be running on battery.

Change-Id: Ic76d966f11e592b9dd671879977bf999ebc11eef