[SystemInfo][Display] Correct DPI property wrong type

The Tizen SystemInfo spec require dotsPerInchWidth and dotsPerInchHeigth as unsigned long, while
it were double that returned. This cause tct-systeminfo-tizen-tests case 173 and case 184 fail.

The fix is simply to convert the value to unsigned long before sending.
Merge pull request #218 from YuZhiqiangX/master

 [SystemInfo][Display] Correct DPI property wrong type