gki: Use shlex.split() to split command line argument strings

Bug: 211741246
Change-Id: I9bd08f77d93c7b2793623dd49511f10e37d0532a
Test: Presubmit unit tests
diff --git a/gki/generate_gki_certificate.py b/gki/generate_gki_certificate.py
index 0765c08..2797cca 100755
--- a/gki/generate_gki_certificate.py
+++ b/gki/generate_gki_certificate.py
@@ -18,6 +18,7 @@
 """Generate a Generic Boot Image certificate suitable for VTS verification."""
 
 from argparse import ArgumentParser
+import shlex
 import subprocess
 
 
@@ -73,7 +74,7 @@
 
     additional_avb_args = []
     for a in args.additional_avb_args:
-        additional_avb_args.extend(a.split())
+        additional_avb_args.extend(shlex.split(a))
     args.additional_avb_args = additional_avb_args
 
     return args