Top level screen for App Fuel Gauge.

Shows sorted list of power usage (with some debug information) by the top 10
apps and subsystems. Doesn't yet take into account certain subsystems such
as data network usage, audio/video DSP usage, Bluetooth and lights other than
screen backlight.

Screen, Idle, Wifi and Voice usage and CPU time are accounted for.

Also need to add detail screens for each item and suggested actions.