uid_cputime: Extends the cputime functionality to report power per uid

/proc/uid_cputime/show_uid_stats shows a third field power for each of
the uids. It represents the power in the units (uAusec)

Bug: 21498425
Change-Id: I52fdc5e59647e9dc97561a26d56f462a2689ba9c
Signed-off-by: Ruchi Kandoi <kandoiruchi@google.com>