Using BOARD_AVB_VBMETA_VENDOR_IMAGE_ARGS for vbmeta_vendor.img

The value of BOARD_AVB_VBMETA_SYSTEM_IMAGE_ARGS and
BOARD_AVB_VBMETA_VENDOR_IMAGE_ARGS are usually the same, e.g.,
--padding_size 4096 --rollback_index 1656979200.

However, in a split build, the value of
BOARD_AVB_VBMETA_SYSTEM_IMAGE_ARGS might be empty.
Using BOARD_AVB_VBMETA_VENDOR_IMAGE_ARGS to correctly
assign the value for vbmeta_vendor.img.

Bug: 232360330
Test: TreeHugger
Change-Id: Ia26816bd309084c3a4c0df6b9f2f3fc5a28bc31a