UsbService: Remove stack-trace message during boot.

This removes a stack trace message during the boot under emulation.

The observers tried to access a null reference when no USB configuration
is supported by the emulated device. So do not start them in this case.

+ Change a Slog.w into a Slog.i since this is an acceptable condition.

Change-Id: I126b7f05c9648e39a060eb0974cc6f4ac7533791