Enable use of ro.sf.lcd_density prop by system user

This allow gralloc to initialize the lcd_density value under certain conditions.

Cheery-pick from nvidia oss :
http://nv-tegra.nvidia.com/gitweb/?p=android/platform/system/core.git;a=commit;h=d56c747707c356d5ecd29f5ab97b0157a13f3b44