AVB: allow no metadata in the generic system.img for project Treble

The generic system.img released from project Treble can't contain any verity
metadata (e.g., vboot 1.0, AVB, or any other implementation) because it's
*generic*. To make any device can boot with it, `avbctl disable-verification`
is introduced to set a new flag AVB_VBMETA_IMAGE_FLAGS_VERIFICATION_DISABLED
in the top-level vbmeta to disable the entire AVB verification process. This
should be done prior to flash the generic system.img. See the following link
for details:

    https://android-review.googlesource.com/#/c/418399/

This CL checks whether AVB_VBMETA_IMAGE_FLAGS_VERIFICATION_DISABLED is
set in the top-level vbmeta. When set, skip verifying the vbmeta structs
against androidboot.vbmeta.{hash_alg, size, digest} because it will be
absent in kernel cmdline. Also, only top-level vbmeta struct is read then
returned by libavb in this case.

Note that another flag AVB_VBMETA_IMAGE_FLAGS_HASHTREE_DISABLED, usually
set by `adb disable-verity`, is used to signal fs_mgr to skip setting up
dm-verity, but libavb still verifies all vbmeta structs. fs_mgr will
also verify all vbmeta structs against androidboot.vbmeta.{hash_alg,
size, digest} from kernel cmdline as well.

Also rename SetUpAvb() to SetUpAvbHashtree() to better fit its usage.
This function will return kDisabled when any of the above two flags is set.

Finally, regardless of which flag is set or not set, we still only allow two
return values from avb_slot_verify():

   - AVB_SLOT_VERIFY_RESULT_OK: it's still possible to get this value
     when any of these flags are set in build time. e.g.,
     BOARD_AVB_MAKE_VBMETA_IMAGE_ARGS=--flags 2

   - AVB_SLOT_VERIFY_RESULT_ERROR_VERIFICATION: in most cases we should
     get this value, because the flags are likely set at run time.

Bug: 62523303
Test: boot device with 'avbctl disable-verification'.
Test: boot device with 'avbctl enable-verification'.
Test: boot device with 'adb disable-verity'.
Test: boot device with 'adb enable-verity'.

Test: build image with BOARD_AVB_MAKE_VBMETA_IMAGE_ARGS=--flags 2, then boot device.
      repeat the above steps to boot device again.

Change-Id: Ie8436f3e0e82c78490208f3b85eac5238a9fdfdb