healthd: break link between BatteryMonitor and BatteryPropertiesRegistrar

Make it easier to later separate out binder-related code.

Add helper functions healthd_battery_update(), healthd_get_property() to
allow these operations without needing references to the BatteryMonitor
object.

Change-Id: Ie584bf53e5178ce0a098d0d940d6c311fdff62d4