Send current battery present state on add callback

BatteryControllerImpl won't send updates to the battery unknown state if
it hasn't changed, but BatteryMeterView can get reinflated on theme
change. This change makes sure we send the current state when callbacks
are added.

Test: atest BatteryControllerTest
Bug: 174959955
Change-Id: I30b4a7b7f55a1f26b6451f23721be3a709ddf894