AVB: decouple vbmeta.img from recovery.img for non-A/B devices

For following cases:
Case 1: A/B devices: no change
Case 2: non-A/B devices, with unsigned recovery image:
    not allowed anymore by mandating BOARD_AVB_RECOVERY_KEY_PATH
Case 3: non-A/B devices, with signed recovery image:
    vbmeta.img should not include ChainPartitionDescriptor of recovery.img,
    otherwise device can not even boot into normal mode if recovery partition
    is damaged

This CL will cause a build break if BOARD_AVB_RECOVERY_KEY_PATH
is not set for non-A/B targets with recovery.img
The following is an example to fix the build break by specifying
AVB signing configs for the recovery.img.

BOARD_AVB_RECOVERY_KEY_PATH := external/avb/test/data/testkey_rsa2048.pem
BOARD_AVB_RECOVERY_ALGORITHM := SHA256_RSA2048
BOARD_AVB_RECOVERY_ROLLBACK_INDEX := $(PLATFORM_SECURITY_PATCH_TIMESTAMP)
BOARD_AVB_RECOVERY_ROLLBACK_INDEX_LOCATION := 2

Also note that libavb in bootloader needs an update to include this
commit Iaa886037edb18c2ff6c60fa2a7f883ab7303ba1a, to support verifying
recovery.img independently (not through vbmeta.img).

Bug: 130351427
Test (Case 3):
  normal   mode: avb_slot_verify(flags=AVB_SLOT_VERIFY_FLAGS_NONE)
  recovery mode: avb_slot_verify(flags=AVB_SLOT_VERIFY_FLAGS_NO_VBMETA_PARTITION)
Test: PYTHONPATH=build/make/tools/releasetools \
    python -m unittest test_validate_target_files
Test: Use a lunch'd target. `atest --host releasetools_test releasetools_py3_test`
Test: validate_target_files.py with Case-3 target files
Change-Id: I2a73252b385fa463b4abd444923a8acc473df0b4