[SystemInfo][CPU] Modify cpu usage calculation algorithm

CPU load in Tizen document and QA's TCT-test should be a double value between 0.0 and 1.0, although in fact the "load" in Tizen API means "cpu usage", not the traditional "cpu load".

The old implementation gives out a value between 0.0 and 1.0*cpu_core_number.

The patch let it pass the TCT-test and comply to Tizen document.