UsbService: Remove stack-trace message during boot. DO NOT MERGE

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: I801f352574716d7868f182bb6e5ee49e5b12e4f1