Show "ro.sf.lcd_density" param only if exists

[jhenrique09]

Some devices (A6020 for example), do not have this parameter set in system.prop (https://github.com/boulzordev/android_device_lenovo_A6020/blob/cm-14.1/system.prop), is set in init https://github.com/boulzordev/android_device_lenovo_A6020/blob/cm-14.1/init/init_A6020.cpp#L73