Reorder assert-max-image-size and AVB signing

This change fixes the following warning message:
  WARNING: out/target/product/$(TARGET_DEVICE)/boot.img approaching \
  size limit (33554432 now; limit 33554432)

This is because AVB signing will increase boot.img to
BOARD_BOOTIMAGE_PARTITION_SIZE, in order to place the AVB metadata at
the end of the partition. We should check max image size first then use
avbtool to sign it.

The max allowed size will be (partition size - AVB_HASH_META_SIZE) when
AVB is enabled.

Bug: 69115400
Test: build and checks there is no size limit warning message
Change-Id: If9545e64a0d1714a2ec2440f9bd38293e7b1ba9a