Build disabled vbmeta.img for aosp_$arch

System images of aosp_$arch are used as GSIs in P, and traditional GSI
users often need a special vbmeta image to disable verity (if AVB is
employed) befrre they flash the GSI.

"BOARD_BUILD_DISABLED_VBMETAIMAGE := true" builds such vbmeta.img

Bug: 79393905
Test: # For arch in arm, arm64, x86, x86_64, do
    $ lunch aosp_$arch; m -j # found vbmeta.img under $OUT

Change-Id: I113006385991a2daab60d3c55dc03f23f1b383b8