fs_mgr: Use libdm for fs_mgr_verity.

This change uses libdm to replace the hand-coded ioctl code in fs_mgr_verity.
Since AVB2 is now preferred to AVB1, this is a surgical change to allow
us to remove fs_mgr_dm_ioctl. Rather than refactor the target string
construction code, we instead provide a DmTargetVerityString class which
passes a raw parameter string directly to DmTable.

Bug: 110035986
Test: AVB1 device still boots
Change-Id: Iad88993a98abb388a12cec9e6d4543dfb4157414