fs_mgr: libdm: add support android-verity target.

The support for android-verity makes it possible for us to test
raw verified filesystem images to be attached to android-verity
target and have it verified by the kernel.

This makes the testing of android-verity device mapper target
much easier as it doesn't _have_ to be used for root mount.

Bug: 72722987
Test:
 $ losetup /dev/block/loop0 /data/local/tmp/verity_fs.img
 $ dmctl create verity-fs android-verity 0 4200 \
   Android:7e4333f9bba00adfe0ede979e28ed1920492b40f 7:0

Change-Id: Ica6bf5c6e1fd758fdb4005fc8a09755f369a8a0f
Signed-off-by: Sandeep Patil <sspatil@google.com>