Adding developer_gsi_keys.mk

OEMs who are willing to support booting the public Developer-GSI
in locked state can add the following line into a device.mk to include
the corresponding public keys into their first-stage ramdisk.

   $(call inherit-product, $(SRC_TARGET_DIR)/product/developer_gsi_keys.mk)

Bug: 144399552
Test: make checks $OUT/ramdisk/avb/.
Change-Id: I8fdefa0fb26c2d385dda1f72503d12e6fc306694