Fix IllegalStateException in Printspooler

Open webpage in chrome(or any thing to print) and click print and then press
back, throws a IllegalStateException.
Currently in onDestroy, doFinish() is called which further calls
unbindService()in PrintSpoolerProvider,but since service has already been
unbinded,it throws a Service not registered error.

Change-Id: If58a951e9e66a4048f0ece3100aabe49cee0ab84