usb: Don't show the notification when no USB device is connected

Currently, the "Charging connected device via USB" notification appears
and then disappears when a USB device is disconnected. The notification
should not appear when no USB device is connected. This CL prevents
this.

Also prevents the notification from showing up when a mouse is
connected.

Bug: 234548453
Test: Peripheral mode, host mode, and Power/Data role swap
Signed-off-by: Jimmy Hu <hhhuuu@google.com>
Merged-In: Ie4c67c639d8633361e5a3675502283cae4e35bd9
Change-Id: Ie4c67c639d8633361e5a3675502283cae4e35bd9