gralloc: Hide dmabuf heap allocator behind of a flag

... so that devices that have API level 31 but do not have the
system dmabuf heap installed (as it is not required) do not spam
error logs before falling back to memfd.

BUG=b:227375257
TEST=build Cuttlefish with aosp/2051732
TEST=cvd start
TEST=vts -m VtsHalGraphicsMapperV4_0Target

Change-Id: I7ab79ce472b1e3d84ffa8bb06044ecc933847656
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/minigbm/+/3563512
Commit-Queue: Jason Macnak <natsu@google.com>
Reviewed-by: Yiwei Zhang <zzyiwei@chromium.org>
Commit-Queue: Yiwei Zhang <zzyiwei@chromium.org>
Tested-by: Yiwei Zhang <zzyiwei@chromium.org>
2 files changed