BatteryStats: Fix regression in wakelock power distribution

We used to give 50% of the cpu time of each app to any app holding a wakelock
while the screen is off.
Since we switched to the new kernel module for measuring app's cpu time, this distribution
was lost.

Bug:21876567
Change-Id: I42c294547f63d150d9929271ca0e27fedaaa9d77