Add support for TARGET_SCREEN_DENSITY

This variable is used to populate the ro.sf.lcd_density property
in /vendor/build.prop, as well as choosing the correct density
bucket for assets in recovery.

Bug: 118676832
Test: inspect taimen and crosshatch in recovery
Test: adb shell getprop ro.sf.lcd_density
Change-Id: I80cf4d28ecfbae17f1ee206a279e7d9144bd7c05