Fix remaining diffs for init_boot and boot images

- Add missing $(cat ) around the fingerprint file
- Add partition size to init_boot
- Add avb keys and algorithms to boot partitions. I used to switch
  between the two modes in bootimg modules based on if a key was set,
  but now that we are setting a key here I added an explicit mode
  property.
- With all these, there are still diffs because avbtool.py introduces
  randomness for the salt. If you patch out the randomness locally
  the diffs come out clean.

Bug: 377562951
Bug: 377563630
Test: m soong_generated_boot_filesystem_test soong_generated_init_boot_filesystem_test
Change-Id: I448dfb431c240e8d06555fcc5d3e02660e8c2bfb