Update GF board variable for dynamic partition

Goldfish specific board variable:

BOARD_EMULATOR_DYNAMIC_PARTITIONS_SIZE

is used to create misc_info.txt, which is needed to mix GSI or CSI on
Goldfish vendor image, and need to reflect the size of the current
super.img, which is now set to 4G+8M (from 3G+8M previously).

Bug: 174442566
Test: $ lunch aosp_x86_arm-userdebug; m
      $ ls -l $OUT/super.img	# make sure it's 4 GB
      $ grep -i dynamic_partitions_group_size $OUT/misc_info.txt
      super_emulator_dynamic_partitions_group_size=4294967296
Change-Id: Idd0fb302b20780ac97959fabec231a632205d46d