Falls back to dialogs when heads-up are N/A.
Add new ongoing notification if battery saver mode
is active. Offer to start battery saver on warnings,
if not already started.
True up BatteryMeterView's levels to the latest threshold
levels. 15% for first warning.
Bug:13329308
Change-Id: Id8ad11a1997079ee7165ae003a8fa1c744462ab3