adb: improve race between device_connected and device_disconnected.

If a device is unplugged while it's being processed, we can be inside
register_usb_transport still when we call unregister_usb_transport,
leading to bad things happening. Move the call of
register_usb_transport into the scope guarded by usb_handles_mutex.

Bug: http://b/62200735
Test: manual
Change-Id: I2517dcb9d30d9a08cebbb5f715ae3fd30c510109