Moving system_other key into product.img

Currently system_other AVB public key is placed in system.img.
However, this makes it's harder to have a *generic* system.img
across different product configs. Moving the key to /product
partition to allow more product-specific AVB keys.

Device board config can add /product/etc/fstab.postinstall,
to mount system_other with this key in /product. It can specify
different mount options, file systems, verity settings, etc., in
this product-specific fstab as well.

Bug: 123611926
Test: `make productimage` checks the following is generated.
      $OUT/product/etc/security/avb/system_other.avbpubkey
      Also checks it's included in $OUT/installed-files-product.{json, txt}

Test: run the following command and checks that
      PRODUCT/etc/security/avb/system_other.avbpubkey is updated:
      ./build/tools/releasetools/sign_target_files_apks \
        --avb_system_other_algorithm SHA256_RSA2048 \
        --avb_system_other_key external/avb/test/data/testkey_rsa2048.pem \
        out/dist/*-target_files-*.zip signed-target_files.zip

Change-Id: I6804f29941bec54375d80bd68a5aedb5c23b842e