fs_mgr: adds/changes some public APIs for early mount in init

Several changes in this CL:
  - Moves class FsManagerAvbHandle to public API
  - Adds a parameter 'wait_for_verity_dev' for FsManagerAvbHandle::SetUpAvb()
    to allow not to wait for verity device gets created
  - Adds FsManagerAvbHandle::AvbHashtreeDisabled() to query whether AVB is disabled
  - Adds fs_mgr_is_avb() to query whether a fstab_rec has MF_AVB flag

Bug: 33254008
Test: test AVB on bullhead
Change-Id: I89c43ca574ae632db8a700fc2590a1f80212c993