avb_ops: support reading from a logical partition

On some devices (e.g., emulator), init needs to read AVB footer from
a logical partition because:

   1) Dynamic/logical partition is enabled
   2) The partition is AVB chained, i.e., need to locate footer from the end
   3) Logical partition is not understandable by bootloader,
      but there is no bootloader in this case

Bug: 125540538
Bug: 128434470
Test: boot and force the fallback path, to check it can get logical path
Change-Id: Ie304bce234cbf0f938f386f7ce59235c851e0e2d