emerald: Use TARGET_SCREEN_DENSITY (#3)

This variable is used to auto-populate the ro.sf.lcd_density as
well as selecting density bucket for resources in recovery.

Bug: 118676832
Test: adb shell getprop ro.sf.lcd_density
Test: boot into recovery
Change-Id: Ib383d18b61e7a2058a69ae26136aa053fdba4df2

Co-authored-by: Anton Hansson <hansson@google.com>