Adding vendor_boot-test-harness.img

boot-test-harness.img is used to allow adb root on user build
images. It also sets properties: ro.audio.silent=1 & ro.test_harness=1.

GKI 2.0 devices will use BOARD_PREBUILT_BOOTIMAGE so
boot-test-harness.img will not be generated.

Therefore, we have to introduce the vendor_boot-test-harness.img
as an alternative for boot-test-harness.img.

In the future, we'll simplify the flow as:
  + If a device has a /vendor_boot partition, builds
    vendor_boot-(test-harness|debug).img.
  + Otherwise, builds boot-(test-harness|debug).img.

boot-(test-harness|debug).img needs to be kept for some clients
to gracefully transit to using vendor_boot-(test-harness|debug).img.

Bug: 194654549
Test: make then `unpack_bootimg --boot_img $OUT/vendor_boot-test-harness.img`
Test: Check the ramdisk content in ./out/vendor_ramdisk

Change-Id: If3a1393b4ff3e69bb9b62f3b843b7858437d47bf