Using a dir /avb for DSU avb keys

Previously the AVB public keys for DSU is hard coded for each key path.
This adds the burden for maintaining the codebase, e.g., need to change
avb_keys each time when any new key is added or some old key is deleted.

With commit Ie74845d8c8e4aa45e8a9e3b862424cec641f8090, it's possible to
load avb keys from a directory. Let's use "/avb" for DSU AVB keys.

Also removing fstab entries that specifying vbmeta_partition. The AVB
keys used in DSU flow should be a separate set from the original
vbmeta.img (or vbmeta_system.img).

The following link has more info about DSU:
  https://developer.android.com/topic/dsu

Bug: 141284191
Bug: 144399552
Test: boot a GSI via DSU

Change-Id: I6bc1e068368e64cb0bbf6ef63805444b35a1fdfb