fs_mgr_avb: refactors how vbmeta is loaded

Adds two classes FsManagerAvbhandle and FsManagerAvbVerifier to replace the
following functions or struct:

    - fs_mgr_load_vbmeta_images() -> FsManagerAvbhandle::Open()
    - fs_mgr_unload_vbmeta_images() -> deleted
    - fs_mgr_setup_avb() -> FsManagerAvbhandle::SetUpAvb()

    - androidboot_vbmeta -> FsManagerAvbVerifier
    - load_vbmeta_prop() -> FsManagerAvbVerifier::Create()
    - verify_vbmeta_images() -> FsManagerAvbVerifier::VerifyVbmetaImages()

And only invokes FsManagerAvbhandle::Open() when there is a fstab entry having
'avb' flag (need HASHTREE descriptor). fs_mgr_is_avb_used() can be
removed as it only checks system property "ro.boot.vbmeta.hash_alg" to
decide whether vbmeta needs to be loaded, which might not be accurate.

For example, there are only HASH descriptors in the verified chain but
no HASHTREE descriptors. In this case, the fs_mgr doesn't have to do
anything because it only takes care of HASHTREE descriptors.

Also adds a new class FsManagerAvbOps to provide the C++ binding
FsManagerAvbOps::AvbSlotVerify() for libavb->avb_slot_verify().

Bug: 33254008
Test: test AVB on bullhead
Change-Id: I8fe15ba01c277152630a2a5c1c5c7f25fbf34030