Fix handling usb dialog for wall chargers.

UsbPort.POWER_ROLE_SINK is orthogonal to the type of the charger
attached. POWER_ROLE_SINK would be the case for AC charging and
USB charging. Therefore query BatteryManager for the charger
type.

Bug: 34972898
Test: Charging notification should not show for pixel-c chargers.
Change-Id: I8dddcd7727b6af973bd173d2c6e325aa4be2ca3a