adb: Prevent registering usb_disconnect twice

adbd can receive multiple AUTH_RSAPUBLICKEY packets. This happens for
example when booting with usb attached when we retry authenticating
after the framework is done booting. Make sure usb_disconnect is only
registered once, otherwise this creates a loop in the disconnects list.

Bug: 8504991
Change-Id: Ia1f9a37005dd17b7eefee1493d622e1679263eea