Allow the qemu.sf.lcd_density property to override the value of ro.sf.lcd_density

ro.sf.lcd_density is usually defined in the build.prop file which is parsed by init
before anything else. Since its name begins with "ro.", this property is write-once
and cannot later be modified, e.g. in /system/etc/init.goldfish.sh.

In other words, you cannot use "emulator -prop ro.sf.lcd_density=<value>", since
it is impossible to override the value defined in build.prop

This patch modifies the system to recognize "qemu.sf.lcd_density" as an override
value, which can be set with "emulator -prop qemu.sf.lcd_density=<value>", forcing
a specific density.

A later patch will allow the emulator to automatically set this property depending
on AVD hardware configuration settings.