fs_mgr_avb: introducing class VBMetaData

Currently vbmeta images are stored in AvbSlotVerifyData defined in
libavb, which contains some fields that isn't needed by fs_mgr.
e.g., loaded_partition, rollback_indexes. Adding a new class VBMetaData
to replace it.

Bug: 112103720
Bug: 117960205
Test: boot crosshatch
Change-Id: I480461dad3c6aca7e028097662a3b06c3aa6646d