Support mixing local system_ext and product images

Acloud supports the use case of shared system images on cuttlefish.
A set of shared system images consists of system.img, system_ext.img,
and product.img. system_ext and product are optional. The images are
mixed into super.img. Verified boot must be disabled for them.

Test: acloud-dev create -vv --local-image --local-instance \
      --local-system-image ~/cf
Test: acloud-dev create -vv --local-image ~/target_files.zip \
      --cvd-host-package ~/cvd-host_package.tar.gz \
      --local-system-image ~/system.img \
      --host 192.168.9.2 --host-user vsoc-01 \
      --host-ssh-private-key-path ~/id_rsa
Test: acloud-dev create -vv --avd-type goldfish --local-instance \
      --local-image ~/img --local-tool ~/emu \
      --local-system-image ~/gsi \
      --local-tool ~/tmp/acloudsys/otatools
Bug: 296249675
Change-Id: I4a3f89fb63008f2dc8270513f1989024e074da46
11 files changed