libdm: Add DmTargetVerity and use it in fs_mgr_avb.

This change adds a DmTargetVerity class and replaces the existing verity
table builder in fs_mgr_avb.cpp. DmTargetVerity must be constructed with
all the required parameters for dm-verity. Additional arguments can be
appended via helper methods.

Bug: 110035986
Test: libdm_test gtest; boots with verity on AVB devices
Change-Id: I4fc2f7ef122c841e8b4ec4156177a6224ffbac30