Use AVB footer to determine caremap

Care maps need to store the original image size, which excludes bytes
used by hash tree or FEC code.

We used to propagate original image size using the global OPTIONS
dictionary. This is bad coding practice, and also fragile because we
have to make sure what's stored in dictionary and what's on disk are
consistent. Instead, let's read the content of images on disk, and parse
the AVB footer. The AVB footer contains the ground truth original image
size.

Test: build OTA, make sure the care maps have valid range
Bug: 246504616
Change-Id: I9250b478ab34dda60578a6b8c23ae6d7a9385788