Consider prebuilt vendor.img as having vendor partition.

vendor.img could be built from the source - in which case we have
unpacked files under VENDOR/ - or dropped in as a prebuilt binary blob.
We should consider either of them as target having a vendor partition.
Because we need to add its AVB hashtree info into vbmeta.img if target
is using AVB. Otherwise libfs_mgr would refuse to mount this
"AVB-enabled" vendor.img.

For targets not using AVB, this change is no-op.

Bug: 65462819
Test: Having vendor.img as prebuilt, `make dist`. Check that the
      generated vbmeta.img contains the info from vendor.img.
Test: Build, flash and boot the above image.zip.
Change-Id: Iaeb30e2059cb33fb39f23e5ffd28f338d00ccbfc
Merged-In: Iaeb30e2059cb33fb39f23e5ffd28f338d00ccbfc
(cherry picked from commit b22afea0)