Support dm-verity with verification based on root digest

Currently the only ways to enable dm-verity were relying on its built-in
vbmeta image or containing its public key on standalone vbmeta image.
Merging this change will support enabling dm-verity based on hashtree
descriptor root digest for standalone vbmeta image.

Bug: 285855436
Test: Presubmit
Test: adb shell /apex/com.android.virt/bin/vm run-microdroid --vendor /vendor/etc/avf/microdroid/microdroid_vendor.img
Change-Id: I51eb64cae2ca8b4e97f1c6419b35d45e6f51cacb