certify_bootimg: support spaces in extra_args

Using shlex.split() to split extra_args in shell-like syntax
instead of str.split().

An usage example:
  certify_bootimg --boot_img boot.img \
      --algorithm SHA256_RSA4096 \
      --key external/avb/test/data/testkey_rsa4096.pem \
      --extra_args "--prop gki:nice" \
      --extra_args '--prop space:"nice to meet you"' \
      --output boot-certified.img

Bug: 223288963
Test: atest --host certify_bootimg_test
Change-Id: I0dd195fa7580016c2e5b4ac8ab0cb4d1ebc74fac
2 files changed