Implement control of auto power save mode.

Follow the setting for auto power save mode in both
battery service and battery UI.  Default level is 15 when
setting is not set; otherwise it is whatever the setting
gives, with 0 meaning auto battery save is off.

Change how we define the "turn off warn" level to be
an adjustment from the warn level, so we can have a good
value for whatever auto setting is set.

Fix power manager to never go in to power save mode when
plugged in, even if the user has manually turned it on.

Add new delete option to settings command, because I needed
it for testing.

Change-Id: I512b691df84399d50b8e751fd50732c6093ebe85